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DYNAMO ELECTRIC MACHINERY; 



ITS CONSTRUCTION, DESIGN, 
AND OPERATION 



DIRECT CURRENT MACHINES 

BY 

SAMUEL SHELDON, A.M., Ph.D. 

PROFESSOR OF PHYSICS AND ELECTRICAL ENGINEERING AT THE POLYTECHNIC 

INSTITUTE OF BROOKLYN, MEMBER OF THE AMERICAN INSTITUTE 

OF ELECTRICAL ENGINEERS, AND FELLOW OF THE AMERICAN 

ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE 

ASSISTED BY 

HOBART MASON, B.S. 

SECOND EDITION, REVISED 



ifflpfi 



NEW YORK: 
D. VAN NOSTRAND COMPANY 
23 Murray and 27 Warren Sts. 
19OI 



T/i ^SI- 
S' 

\°[0\ 



THE LIBRARY «F 

OONGKESS, 
Two Con* Receive* 

FEB. 20 1902 

OWYRMMT ENTRY 

)LASS ^J©6fc N». 



OLA 



copy a. 



Copyright, igoo, by 
D. VAN NOSTRAND COMPANY 












PREFACE. 



This book is intended to be used primarily in connec- 
tion with instruction on courses of electrical engineering in 
institutions for technical education. It is laid out on the 
lines of the lectures and the instruction as given in the 
Polytechnic Institute of Brooklyn. It is intended equally 
as much for the general reader, who is seriously looking 
for information concerning dynamo electrical machinery of 
the types discussed, as well as a book of reference for 
engineers. 

The first two chapters are devoted to a brief but logical 
discussion of the electrical and magnetic laws and facts 
upon which the operation of this class of machinery 
depends. Calculus methods have been employed in a few 
places in these chapters, but the results arrived at by use 
of them are in such a form that they can be utilized by the 
reader who is unfamiliar with the processes of the calculus. 

In the chapter on design it has seemed advisable to 
express the flux density in lines per square centimeter. 
Both the square centimeter and the square inch are used 
in practice. The alteration of the formulas to square inch 
units is obviously simple. 

We wish to express our thanks to the various manufac- 
turing companies who have so courteously given informa- 
tion, and who have kindly loaned electrotypes of their 
apparatus. 



PREFACE TO THE SECOND EDITION. 



The cordial reception accorded this volume upon its 
appearance has resulted in a rapid exhaustion of the edi- 
tion. Its adoption as a text-book by many educational 
institutions has convinced the author that others concur 
with him in his judgment as to what should be embodied 
in such a book ; and he is encouraged in the preparation of 
a second volume which will treat of alternating current 
machines and which will appear shortly. Such errors as 
had inadvertently crept into the first edition are here cor- 
rected, in so far as they have been brought to the atten- 
tion of the author. 



CONTENTS, 



CHAPTER PAGE 

I. Electrical Laws and Facts i 

II. Magnetic Laws and Facts 12 

III. Armatures 31 

IV. Field Magnets 67 

V. Operation of Armatures 77 

VI. Efficiency of Operation 92 

VII. Constant Potential Dynamos 103 

VIII. Constant Current Dynamos 129 

IX. Motors 161 

X. Series Motors 185 

XI. Dynamotors, Motor-Generators, and Boosters . . 208 

XII. Management of Machines . 218 

XIII. The Design of Machines 232 

XIV. Tests 251 



DYNAMO ELECTRIC MACHINERY. 



CHAPTER I. 

ELECTRICAL LAWS AND FACTS. 

i. Mechanical Units Force is that which tends to 

produce, alter, or destroy motion. The units of force are 
the pound and the dyne. The dyne is that force, which 
acting on one gram for one second, will produce a velocity 
of one centimeter per second. 

Work is the production of motion against resistance. 
The units of work are the foot-pound and the erg. The 
foot-poimd is the work done in lifting a body weighing one 
pound one foot vertically. The erg is the work performed 
by a force of one dyne in moving a body one centimeter 
in the direction of its acting. The joule is a larger unit 
much used, and is equal to io 7 ergs. 

Energy is the capacity to do work. Energy is divided 
into Kinetic energy and Potential energy. A body pos- 
sesses kinetic energy in virtue of its motion, while poten- 
tial energy is due to the separation or the disarrangement 
of attracting particles or masses. A wound up spring has 
potential energy because of the strained positions of the 
molecules, while a weight raised to a height has potential 
energy because of the separation of its mass from the 



2 DYNAMO ELECTRIC MACHINERY. 

attracting mass of the earth. The potential energy of a 
body is measured by the work required to put the body 
into its strained condition. Kinetic energy is measured by 
the product of the weight into the square of the velocity 
divided by twice the acceleration due to gravity, or 

Kinetic Energy = . 

2 g 

Power is the rate of performance of work. Its units 
are the horse-power and the watt. A horse-power is 33,000 
foot-pounds per minute. A watt is io 7 ergs per second. 
One horse-power is equivalent to 746 watts. The number 
of watts in an electrical circuit carrying a certain number of 
amperes of current under a pressure of a certain number 
of volts is expressed by the product of the amperes into the 
volts. If we let T equal the torque or twisting moment 

and (o equal the angular velocity (= — — where n is the 

number of revolutions per minute), then the horse-power 

60 (oT 2 -KtiT 
33000 33000 ' 

In a belt-driven machine the torque in the shaft is equal 
to the difference in tension of the two sides of the belt 
multiplied by the radius of the pulley in feet, hence 
T= (F- F f ) r. 

2. Absolute and Practical Units Since distinction 

must continually be made between the absolute units and 
the practical units, throughout this work the capital letters 
/, E, and R will be used for quantities expressed in prac- 
tical units, the ampere, the volt, and the ohm ; the lower 
case letters i, e, and r for quantities expressed in absolute 
units of current, pressure, and resistance respectively. 



ELECTRICAL LAWS AND FACTS. 3 

The absolute unit of current is such that, when flowing 
through a conductor of one centimeter length, which is 
bent into an arc of one centimeter radius, it will exert a 
force of one dyne on a unit magnet pole (§ 10) placed at the 
center. 

The absolute unit of difference of potential exists between 
two points when it requires the expenditure of one erg of 
work to move a unit quantity of electricity from one point 
to the other. This unit of quantity is the quantity which, 
in a second, passes any cross-section of a conductor in 
which a unit current is flowing. 

The absolute unit of resistance is offered by a body when 
it allows a unit current to flow along it between its two 
terminals, when maintained at a unit difference of potential. 

Current, 7= — i. 
10 

E.M.F., E = io 8 *. 
Resistance, i? = 10V. 

It is convenient and rational to make a distinction be- 
tween electromotive force and difference of potential. 
Electromotive force is produced when a conductor cuts 
magnetic lines of force, or when the electrodes of a pri- 
mary battery are immersed in a solution. But a difference 
of potential may exist merely because of an electric cur- 
rent. Between any two points of a conductor carrying a 
current there is that which would send a current through 
an auxiliary wire connecting these points, and we call it 
difference of potential. If the current in the original con- 
ductor be doubled, the difference of potential between the 
same two points will be doubled, showing that this differ- 
ence of potential exists because of the current flowing in 



4 DYNAMO ELECTRIC MACHINERY. 

the original conductor. The word pressure is used for 
either difference of potential or for E.M.F. with obvious 
relevancy. 

3. Ohm's Law Ohm's law is expressed by the for- 
mula „ 

hi 

where / is the number of amperes flowing in an undivided 
circuit, E the algebraic sum of all the electromotive forces 
in that circuit, and R the sum of all the resistances in 
series in that circuit. 

The form of the equation E = IR, as applied to a por- 
tion of a circuit, is much used under the name of Ohm's 
law. In this case, however, E is not E.M.F., but differ- 
ence of potential, as explained in the last article. 

If, in a house lighted by electricity, the service maintains 
a constant pressure of 100 volts at the mains where they 
enter from the street, and no lights be turned on, then at 
every lamp socket in the house there will be a pressure of 
100 volts. If now a lamp be turned on, it will be working 
on less than 100 volts, because of the drop or fall of po- 
tential. If many lamps be turned on, a considerable drop 
may occur. The drop is caused by the resistance of the 
wires carrying the current from the place of constant po- 
tential to the place where it is used, and the volts lost have 
been consumed in doing useless work in heating the wires. 
That the drop is proportional to the current flowing is 
shown by a simple application of Ohm's law. 

Let R be the resistance of the line, and E d the volts 
drop caused thereby when a current I flows. Then 

E d = IR, 
from which it is evident that the drop varies as the cur- 
rent when the resistance in the line is constant. 



ELECTRICAL LAWS AND FACTS. 5 

4. Resistance of Conductors. — The resistance R of a con- 
ductor is expressed by the formula R = — , where o- is a 

constant called the resistivity, and depending upon the 
material and the temperature of the conductor, /is the 
length in centimeters, and A the cross-section in square 

cms. The reciprocal of the resistivity, — is called the con- 
ductivity of a substance. 

The conductivity of copper depends on its purity, and on 
its physical condition, soft copper having 1.0226 times the 
conductivity of hard copper. Lake copper has a high con- 
ductivity because of its pureness. The same is true of 
electrolytic copper. This latter is now very largely used, 
though for a while there was a prejudice against it because 
it was said to be brittle. Temperature affects the resist- 
ance of metals. In pure metals the increase of resistance 
for a rise of i° C. is about .004 times their resistance at 
o° C. Various alloys of iron, nickel, and manganese have 
a high value for <r, and do not have so high a temperature 
coefficient as given above. Iron heated in contact with 
copper gives a large thermal E.M.F., which militates against 
its alloys being used for resistances in measuring instru- 
ments. 

If in the foregoing expression for R the centimeter and 
square centimeter be the units of length and cross-section 
respectively, then the following list gives the value of <r for 
various metals in microhms (1 microhm = T(J ]jViyu^ ohm). 

Copper . . . .at o°C, *-594 

Iron " " 9.5 

Steel " " 13.0 

1 8$ German Silver " " 27. 

30% " " 45- 



6 DYNAMO ELECTRIC MACHINERY. 

A circular mil is a circle j-^^ inch in diameter, and a 
wire one foot long and one circular mil cross-section is 
called a mil-foot. The resistance of a mil-foot at o° C. of 

Copper = 9.59 ohms, 
Iron =58. ohms, 
Steel = 82. ohms. 

The American Institute of Electrical Engineers has 
adopted as its standard resistivity for soft copper one given 
by Matthiessen. A wire of standard soft copper, of uni- 
form cross-section, of one meter length, and weighing one 
gram, should have a resistance of o. 141 729 international 
ohms at o° C. A commercial copper showing this resis- 
tivity is said to have 100 per cent conductivity. Copper is 
frequently found having a conductivity of 102 per cent. It 
is in these cases almost invariably electrolytic copper. 

5. Insulating Materials. — Materials which are to be 
used for insulating from each other the various electrical 
circuits of dynamo electric machines should possess the 
following properties : — 

They should have a high insulation resistance, and this 
resistance should be maintained high over the range of 
temperatures to be found in machines. They should 
furthermore have a dielectric strength sufficient to pre- 
clude any possibility of their being perforated by the 
voltages liable to exist between the conductors which they 
separate. This strength must also exist throughout all 
probable ranges of temperature. They must possess such 
physical properties as will permit of mechanical manipula- 
tion, as they must be oftentimes bent and twisted. Of 
course the chemical constitution should not be altered by 



ELECTRICAL LAWS AND FACTS. 7 

any change of temperature to which they would be sub- 
mitted. 

Mica possesses the highest insulation resistance and the 
largest dielectric strength to be found. It requires 1000 
volts to perforate a sheet 1 mil in thickness. Its chemical 
constitution is unaffected by high temperatures. It is 
not, however, mechanically strong. 

Preparations of fibrous materials with linseed oil, which, 
after being dried, have been thoroughly baked, are fairly 
good insulators. As water is generally present in their 
pores, their insulation resistance, upon heating, decreases 
until the temperature has reached ioo° C, and then it in- 
creases. These preparations are mechanically flexible. 
Preparations of fibrous material with shellac are good in- 
sulators, but crack upon bending. 

Vulcanized fibers are made by treating paper fiber chemi- 
cally, and, when dried, they have a fairly high insulation 
resistance, but they readily absorb moisture, and, upon dry- 
ing, are liable to warp and twist. They furthermore be- 
come brittle when heated. 

Sheets of insulation made up from pieces of scrap mica 
cemented together by linseed oil or preparations of shellac, 
when carefully constructed with lapped joints, exhibit 
nearly as good insulating and dielectric properties as sheet 
mica.. While not perfect mechanically, these sheets permit 
of bending better than pure mica. 

Vulcabeston, which is a preparation of asbestos and rub- 
ber, exhibits fairly good insulating and mechanical quali- 
ties, and is especially fitted for higher temperatures. Its 
dielectric strength is about T ^ of that of mica. 

6. Divided Circuits If a current / be flowing through 



8 DYNAMO ELECTRIC MACHINERY. 

R, the undivided part of the circuit shown in Fig. I, and 
if I x and 7 2 be the currents flowing in the shunt resistances 
R 1 and R 2 , then I ' = I x + I 2 , and, since the pressure E 
upon each shunt is the same, by Ohm's law, 

/i = | and/2 = | 

The currents in the brandies of a divided circuit are in- 
versely as the resistances of the branches. 

If R e be a single resistance, that substituted for the 
shunted resistances R x and R 2 will leave I unchanged, then, 
by Ohm's law, 

E _E 2? 

R e R 1 R 2 

r 

R\ R2 



R. + R* ^r+^r 

The resistance equivalent to a number of shunted resistances 
is equal to the reciprocal of the sum of the reciprocals of the 
separate resistances. 

Rl 

ti 



t 




Fig. 1. 

7. Power of Electric Current A difference of poten- 
tial e between two points requires e ergs of work to bring 



ELECTRICAL LAWS AND FACTS. 9 

a unit quantity of electricity from one point to the other. 
A unit of quantity is one absolute unit of current flowing 
for one second. Hence a current i flowing for t seconds with 
a difference of potential e does eit ergs of work. Likewise 
a current / flowing t seconds gives It coulombs of quantity, 
and with a difference of potential of E volts does Eit X io 7 
ergs of work. Hence the work per second or the power 
is EI x io 7 absolute units. The practical unit of power is 
the watt, and equals io 7 absolute units. Hence, remember- 
ing that by Ohm's law E = IR, the power of a current in 

Watts = EI = PR. 

For commercial currents and voltages the watt is a need- 
lessly small expression, hence the kilowatt (= 1,000 watts) 
is generally used as the unit of electrical power. It is rep- 
resented by the abbreviation k.w. The horse-power is 
equal to 746 watts, or approximately three-fourths of a k.w. 

8. Heat Developed by a Current When a current / 

does work in overcoming a resistance R, the work per- 
formed is converted into heat. By the last article the 
work thus done per second, or the power expended, will be 
PR watts. Since this rate of production of heat is often 
of no service, this expenditure of power is generally called 
the PR loss. 

This production of heat causes a rise of temperature in 
the conductor, and the temperature will continue to rise till 
the heat generated per second by the PR Ins is exactly 
counterbalanced by the rate of dissipation of heat by con- 
duction, convection, and radiation. 

The necessary resistances of electrical machines involve 



10 DYNAMO ELECTRIC MACHINERY. 

the production of heat in their operation (as does also fric- 
tion and reversal of magnetism), which causes a rise of tem- 
perature. As insulating materials can survive only moder- 
ately high temperatures, such machines must be designed 
to operate without becoming too hot. This is accomplished 
by decreasing the PR loss, by increasing the radiating sur- 
face, and by supplying ventilation. 

9. Fuses. — These are devices intended to protect cir- 
cuits from destruction or damage due to an excessive flow 
of current through them. They protect them by being 
themselves destroyed. They are generally made of lead 
or alloys of lead. Lead is liable to become oxidized after 
having been installed for some time. It is then liable to 
form a tube of hard oxide, which is sufficiently strong to 
hold molten lead in its interior, so as to maintain an elec- 
trical contact in the circuit which should be broken. Some 
alloys are not open to this objection. These alloys, in 
the form of wires, strips, or ribbons, are fastened at each 
end to copper terminals which are slotted to fit into fuse 
receptacles. The wire with its terminals is called a fuse 
link. Such a link is shown in Fig. 2. 





Fig. 2. 

Copper wires are sometimes used as fuses on trolley cars, 
but the high melting point of copper prohibits its use as a 
protective device on house circuits. 

The current which will fuse a wire of lead alloy depends 
in magnitude upon the length of the wire. Short lengths 



ELECTRICAL LAWS AND FACTS. II 

of a wire of given cross-section and given material will 
carry stronger currents than longer lengths. The heat 
which is generated in the short ones escapes more rapidly, 
owing to the larger masses of metal commonly forming the 
terminals of the fuse. Fuses are rated to carry a given 
amperage, and the rating is stamped upon the copper termi- 
nals. According to the national code the fuses must, how- 
ever, be able to carry indefinitely without melting such a 
number of amperes that the rated capacity is but 80 per 
cent of it. This permits the fuse to carry without melting 
25 per cent above the rated capacity. 




Fig. 3. 



For high voltages, and for large amperages, inclosed 
fuses are sometimes used, in which the fusible conductor is 
surrounded by a packing of finely divided powder in which 
borax is included as an element most desirable. Such a 
fuse is shown in Fig. 3. 



12 DYNAMO ELECTRIC MACHINERY. 



CHAPTER II. 

MAGNETIC LAWS AND FACTS. 

10. Strength of Magnet Pole A unit magnet pole is 

one which will repel an equal like pole, when at a distance 
of one centimeter, with a force of one dyne. 

It follows from this definition that a pole m units strong 
will repel a like unit pole with a force of m dynes. The 
force exerted between two magnetic poles varies inversely 
as the square of the distance between them. Hence the 
force exerted between two magnetic poles of strengths in 
and m' when d centimeters apart is defined by the equation 

_ m m' 

ii. Intensity of Magnetic Field. — A magnetic field is 
of unit strength or intensity when a unit magnet pole placed 
therein is acted upon by a force of one dyne, or when a 
magnet pole m units strong is acted upon by a force of 
m dynes. The strength of a field is usually represented 
by JC. 

12. Magnetic Field and Lines of Force. — The space 
around a magnet where its action is felt is termed the field 
of that magnet. This field may conveniently be consid- 
ered as permeated by lines of force. These lines represent 
the direction of the force exerted by the magnet, and by 
their closeness to each other show the magnitude of this 
force. 



MAGNETIC LAWS AND FACTS. 13 

The student must not get the impression that, because 
the lines spread apart, a point in the field could be chosen 
where there would be no line. These lines may well be 
considered as tubes or pencils of force, filling all the "space 
around the magnet. 

The lines of force contained in any plane passed through 
the magnet pole compose a magjietic spectrum, which can 
be made visible by the familiar experiment of sprinkling 
iron filings on a paper, which is laid over a magnet, and by 
gently tapping it. 

By convention one line of force per square centimeter is 
considered to represent a field of unit strength, the square 
centimeter being so taken that it is at all points perpendic- 
ular to the lines cutting it. Hence the strength or inten- 
sity JC of any field can be expressed by the number of 
'lines of force per square centimeter. 

Suppose a sphere of one centimeter radius to be circum- 
scribed about a unit magnet pole. Another unit pole at 
any point on the surface of this sphere will be acted upon 
by a force of one dyne. Hence there exists a unit field at 
any point on this surface. But there are 4 -k square centi- 
meters on this surface, and each square centimeter will 
be cut by one line of force. Therefore, tJiere emanate 
from a unit magnet pole 4. it lines of force. Similarly a 
magnet pole of strength m sends out 4 -rr m lines of force. 

A magnetic field is said to be uniform when it has the 
same 3t at every point therein, or when the lines of force 
are parallel. 

13. Electro-Magnetic Induction. — In 1831 Faraday and 
Henry independently discovered that when a conductor was 
moved in a magnetic field, an electromotive force was set 



M 



DYNAMO ELECTRIC MACHINERY. 



up in the conductor. This phenomenon is the foundation 
of all modern electrical engineering. 

An absolute unit of E.M.F. is produced when a conduc- 
tor cuts one line of force per second. If the conductor 
cuts two lines in the second, or one line in half a second, 
then two units are produced. 

If, in the short interval of time, dt seconds, d<f> lines be 

cut, then during that interval the value of the induced 

E.M.F. will be 

d<j> 

or, 

i d$ 



E = 



10 dt 



volts. 



The negative sign is used because the induced E.M.F. 
tends to send a current in such a direction as to demag- 
netize the field. When of no con- 
sequence the negative sign will 
hereafter be omitted. 

If a conductor, Fig. 4, / centi- 
meters long moves parallel to itself 
with a uniform velocity of v cen- 
timeters per second across a uni- 
form magnetic field of strength 
3C, its path making an angle a with 
the direction of the lines of force, 
then the number of lines cut per 
second is 3C/v sin a, and since the 
rate of cutting is uniform, the E.M.F. at any instant is 
e = 3C/v sin a. 
If there be a non-uniformity in the rate of cutting lines, 
due either to an uneven field or an irregular motion, then 




Fig. 4- 



MAGNETIC LAWS AND FACTS. 1 5 

the average value of the induced E.M.F. associated with 

the cutting of <£ lines in the time, t seconds, will be e av =— • 

For suppose the time t to be divided into / equal and small 
periods havir g a duration of A/ seconds. Furthermore, 
suppose that during these successive periods A<£', A<£", A<£'", 
etc., lines be cut respectively. Then the induced E.M.F.' s 
during these periods, which may be represented by /, e" y e' n ', 
etc., respectively, will be as follows : — 



/ 


= 


A?' 


e" 


= 


A<£" 
A/ ' 


e r " 





A<£'" 



A/ 



Adding these / equations, and then dividing by /, gives the 
equation above, viz., 

e av = :• - or E av = —^ volts. 

The average value of the induced E.M.F. is therefore inde- 
pendent of the magnitude of the instantaneous values. 

If a loop of wire revolve, uniformly or otherwise, in a 
magnetic field which is uniform or otherwise, its sides cut 
lines of force at various rates. The instantaneous E.M.F. 
in the whole loop will be as before. 



e = -4> 
dt 

where <£ is the number of lines that links with, or that 
passes through, the loop. If the loop be of n turns, then 



i6 



DYNAMO ELECTRIC MACHINERY. 



the pressure will be n times as great, or during the inter- 
val dt y 

nd ' <f> 



E = - 



oV/ 



14. Direction of Induced E.M.F. — The direction of 
flow of a current induced in a closed circuit by mov- 
ing it in a magnetic field is best represented by drawing 
the conventional representation of the three dimensions 
of space. If the flux be directed upwards, and the motion 
of the conductor be to the right, then the E.M.F. will tend 
to send a current toward the reader. If any one of these 
conditions be changed it necessitates the change of one of 
the others, and conversely the change of any two leaves 







Fig. 5. 




the third unaltered. About the same idea is represented 
in Fleming's Rule, which is as follows : — 

Let the index finger of the right hand point in the di- 
rection of the flux, and the thumb in the direction of the 



MAGNETIC LAWS AND FACTS. 17 

motion. Bend the second finger at right angles with the 
thumb and index finger, and it will point in the direction 
of the E.M.F. 

Another rule is : — 

Stand facing a north magnetic pole. Pass a conductor 
downward. The current tends to flow to the left. 

15. Inductance. — Nearly every electrical circuit which 
has a current flowing in it has lines of force linked with it,, 
due to that current. When the circuit is opened the dis- 
appearance of the lines is accompanied by a cutting of the 
circuit by those lines, and the cutting results in the pro- 
duction of an E.M.F. This is called the E.M.F. of self- 
induction. Its magnitude is dependent upon the rapidity 
with which the field disappears, and upon a constant deter- 
mined by the geometric shape of the circuit and the char- 
acter of the medium in which it is placed. This constant 
is called the self -inductance or the coefficient of self-induc- 
tion of the circuit. It is generally represented by the< 
letter L, and is that coefficient by which the time rate of- 
change of current in the circuit must be multiplied in order * 
to give the E.M.F. induced in the circuit. Its absolute - 
value is numerically represented by the number of lines of 
force linked with the circuit per absolute unit of current in 
that circuit. Its practical unit is io 9 times as large as the 
absolute unit, and is called the henry. In a given circuit it 
varies as the square of the number of turns of wire. Two 
circuits may exercise a mutually inductive action upon each 
other, and an E.M.F. may be induced in one by a change 
of current in the other. This is called the E.M.F . of 
mutual induction. In magnitude it depends upon the shape 
and position of the two circuits, and upon the character 



18 DYNAMO ELECTRIC MACHINERY. 

of medium in which they are placed. It is also dependent 
upon a constant which is called the mutual inductance or 
coefficient of mutual induction of the two circuits. It is 
generally represented by the letter M. It is that coeffi- 
cient by which the time rate of change of the current in one 
of the circuits is multiplied in order to give the E.M.F. 
induced in the other circuit. Its absolute value is numeri- 
cally equal to the number of lines of force linked with one 
of the circuits per absolute unit of current in the other cir- 
cuit. Its practical unit is the same as the practical unit 
of self -inductance, that is the henry, and is io 9 times as 
large as the absolute unit. The coefficient of mutual in- 
duction varies directly as the number of turns of wire in 
either circuit. 

16. Quantity of Electricity Traversing a Circuit Due 
to a Change of Flux Linked with it. — In many dynamo 
tests, and in many magnetic investigations, it is necessary 
to measure, generally by means of a ballistic galvanometer, 
the quantity of electricity traversing a circuit due to a 
change of flux linked with it. If the circuit have a resist- 
ance of r and in dt time the flux linked with n turns 
changes by d<f>y then the instantaneous current 

._~df 
r 

But the quantity, dq = idt, hence 

, ndcf> 
r 

which is independent of time. So if the flux change from 

4> 1 to <£.„ then 

</>! — <f> 2 
q = », 



MAGNETIC LAWS AND FACTS. 19 

or Q — -^— ^ — - microcoulombs. 

100 R 

17. Work Performed by a Conductor Carrying a Current 
and Moving in a Magnetic Field. — Let a conductor carry- 
ing a constant current i be moved in a direction perpen- 
dicular to itself and to the lines of force of a magnetic field. 
Suppose it to move for dt seconds, and in that time to cut 
d<f> lines of force. Then the induced E.M.F. e will be 

<p y| ie q Uan j-it-y f electricity dq that has to traverse 

dt 
the circuit against this E.M.F. during the time dt will 
be idt. Since potential is a measure of work, the work 
required to carry dq units of electricity against a difference 
of potential e is edq ergs. Hence the work in ergs, 

dw = edq = idt X -7- = id<\>. 

Therefore the current i, in cutting <f> lines of force, per- 
forms the work 

w = i<j> ergs. 

From this it is seen that the work done by a conductor 
carrying a current and cutting lines of force is independent 
of the time it takes to cut them. 

In the above discussion, if the field be not uniform or 
the motion be not uniform, the value of e will not be the 
same for each instant of time. But since the result 
obtained is independent of time, it is immaterial how the 
lines are arranged, and how the rate of cutting varies. 

18. Magnetic Potential. — The magnetic potential at 
any point is measured by the work required to bring a unit 
magnet pole up to that point from an infinite distance. 



20 DYNAMO ELECTRIC MACHINERY. 

The difference of magnetic potential between any two 
points is measured by the work in ergs required to carry a 
unit magnet pole from one to the other. The difference 
of magnetic potential is a measure of the ability to send 
out lines of force, or to set up a magnetic field. 

19. Magnetomotive Force of a Circular Circuit Carry- 
ing a Current. — A thin circular conductor carrying a cur- 
rent forms a magnetic shell. If a unit magnet pole be 
taken from the top side of a shell, and carried around to 

the bottom side, work must be 
done, and this work is a measure 
of the difference of potential be- 
tween the two sides of the shell. 
It is immaterial whether the 
pole be carried from one side of 
the shell to the other, or the 
shell be turned bottom side up 
around the pole. In the latter case it is clear that all the 
lines emanating from the pole will be cut once, and once 
only, by the conductor, wherefore 4^ lines will have been 
cut. 

If current i flows in the conductor, then, by § 17, 

Work in ergs i<f> = 4 ni. 

If there be 11 turns of the conductor, each line of force 
will be cut 11 times, and the work will be 4^7/2 ergs. 

Hence the difference of potential between the two sides 

of a thin magnetic shell is 4 tcni or — — 

10 

47T 

In this expression — is a constant, and it is convenient 
to regard nl as a single variable. In connection with it the 




MAGNETIC LAWS AND FACTS. 21 

term ampere-turns is employed, and this is frequently 
written ;//. 

Here the same argument holds as in § 1 7, that the inten- 
sity of field and the rate of cutting lines will vary as the 
pole is in different parts of the path. But the total num- 
ber of lines cut is the same in any case, so the expression 
for work and potential is true, no matter what path the pole 
takes. 

20. Force Exerted on a Field by a Conductor Carrying a 
Current. — When a conductor moves in a field perpendicu- 
lar to itself and to the lines of force, then, from §17, 

Work = icf> ergs. 

If the conductor be / centimeters long, and traverses a dis- 
tance of s centimeters through a uniform field of strength 
3C (3e lines per sq. cm.), then 

$ = ls3C } 
and the 

Work == Us OC ergs. 
But 

Work = force X distance = Fs = ilsSC. 

.-. F = // JC = — dynes. 

10 

21. The Solenoid. — A uniformly wound, long, straight 
coil, carrying a current i, produces a uniform field 3C at its 
center. This coil is called a solenoid, and may be consid- 
ered as composed of magnetic shells arranged at equal dis- 
tances from each other. It takes 471-2' ergs to move a unit 
magnet pole from one side of a shell to the other (§ 19), 
and ^irin ergs to pass it through the n consecutive shells 



22 DYNAMO ELECTRIC MACHINERY. 

of the solenoid. If these n shells occupy a length on the 
solenoid of / centimeters, then 

Work = force X distance = 3C/ = ^irin ergs, 

and the magnetizing force, that is, the strength or intensity 
of field, in the solenoid is 

IO / 

22. Permeability. — The same difference of magnetic 
potential between two points will produce more lines of 
force in iron than in air. Iron is then said to be more per- 
meable than air, or to have a greater permeability. If a 
difference of magnetic potential could set up, at a certain 
place, a field of strength 5C, with air as a medium, and one 
of strength (ft, with some other substance as a medium, 

then the ratio — expresses the permeability of that sub- 

stance. This ratio is usually represented by fx. As 5C 
varies directly with the magnetic difference of potential, it 
becomes a measure of it. Therefore 5C is called the mag- 
netizing force and (ft tlie flnx density, the magnetic density, 
or the induction per square centimeter. For air, vacuum, 
and most substances n = I. For iron, nickel, and cobalt fx 
has a higher value, reaching, in the case of iron, as high 
as 3000. Bismuth, phosphorus, water, and a few other sub- 
stances, have a permeability very slightly less than unity. 

A substance for which fx = o would insulate magnetism. 
There is no such substance. 

The total magnetic flux, </>, which passes through an 
area of A square centimeters, in which the magnetic density 
is ®, is represented by the equation 

cf> = A®>. 



MAGNETIC LAWS AND FACTS. 



23 



The permeability of air is constant for all magnetizing 
forces. This is not the case with iron and other substances 
which have a permeability greater than unity. The value 
of /x, (B, and 3C, which are connected by the relation (B = /x3C, 
are given in the following table for average commercial 
wrought iron, for cast iron, and for steel. The relations 
which exist between (B and 3C are also shown in Figs. 8, 9, 
and 10. These curves are technically known as (B-3C 
curves. 

DATA FOR (B-3e CURVES. 

AVERAGE FIRST QUALITY METAL. 



3C 


Ampere- 
turns 
per Cen- 


Ampere- 
turns 


Wrought and 
Shebt Iron. 


Cast Iron. 


Cast Steel. 




Kilo- 




Kilo- 




Kilo- 




timeter 
Length. 


Length. 


(B 


lines 

per 

Sq. In. 


(B 


lines 

PER 

Sq. In. 


(B 


lines 

PER 

Sq. In. 


10 


7-95 


20.2 


1 1 800 


74 


3900 


25.2 


12000 


77 


20 


15.90 


4O.4 


14000 


90 


55oo 


3.S-S 


13800 


89 


30 


23.85 


60.6 


15200 


98 


6500 


42.0 


14600 


94 


40 


31.80 


80.8 


15800 


I02 


7100 


45-7 


15400 


99 


5° 


3975 


IOI.O 


16400 


106 


7700 


49-5 


16000 


103 


60 


47.70 


121. 2 


16800 


108 


8200 


53-° 


16400 


106 


80 


63-65 


I6I.6 


17200 


III 


8900 


57-2 


16700 


108 


100 


79-5° 


202.0 


17600 


114 


9300 


60.0 


17600 


113 


125 


99.70 


252.5 


17800 


us 


9700 


62.4 


18200 


117 


J 5° 


119.25 


303-0 


18000 


116 


IOIOO 


65.8 


18600 


120 



3£= 1.258 (;//per cm.) = .495 («/per in.). 



•155 (<t> per sq. in.). 



23. Things Which Influence the Shape of the (B-JC Curve. 

— In general all substances mixed with or alloyed with iron 
lower its permeability. In steel and cast iron the per- 
meability seems to be in inverse proportion to the amount 
of carbon present. Carbon in the graphitic (not combined) 
form lowers the permeability less than carbon when com- 
bined. In cast iron and cast steel such substances as tend 
to give softness and greater homogeneity to the metal 



2 4 



DYNAMO ELECTRIC MACHINERY. 



WROUGHT AND SHEET IRON 



711 per centimeter 16 



lit p er 



Permeabilit y lb 



40 50 GO 70 



120 


20000 
18000 
16000 
H000 

03 






























































110 






























































103 
























































1 
























































1 








































































/ 














„ 














































/ 


















[Sfen 




































a 


12000 
10000 
8000 
6000 
4000 
2000 




/ 
























[4/ 


*> 




































/ 


























































Go 






































































































\ 






















o2 










































\ 


















































































30 






































































































































































/ 


















































































Ad 


, 






























































/ 





























































90 100 110 120 



-JO 



Fig. 8. 



CAST IRON 



65 


10000- 






S 




























































\ 
































































\ 
































































\ 
































































\ 
































































V 


s 












































£*" 


(B - 












/ 




\ 












































5 










/ 








\ 










































1 










/ 












\%. 






































c 








/ 
















X 


%< 




































k 32.5 






/ 


' 


















s 


fe 


































m* 






/ 






















\ 


\ 
































.2 






/ 


























S 


































/ 






























\ 


» 






























/ 
































*> 


s 


























2000- 


J 




































S 






















13 








































\ 
























/ 






































) 
























/ 


































• 


• 
























/ 
































































9C 100 110 120 130 140 15 ( TC 



III per centimeter 


16 


32 


43 


64 


80 


96 


112 




nJ per inch 


40 


80 


120 


160 


200 


240 


2S0 




PenneabilityU 


100 


200 


300 


4P0 


500 









Fig. 9. 



1 



MAGNETIC LAWS AND FACTS. 



25 



CAST" STEEL 

























































































































































































































































\ 


\ 




















































































































































































/ 


























































/ 




















"~> 


PB- 




































/ 
























F^Q 


























/ 




























































/ 




























































/ 
















































* 












































































































/ 
































































































































































































































































































































































5 10 15 20 25 30 3 


5 4 


J 43 50 ij I 


65 70 7 



10000 
14000 

(B 

12000 

10000 

8000 
6000 
4000 
2000 



TV I per centimeter 



5JC 



Til per inch 



Permeability a 



Fig. 10. 

when present in limited amounts, say 2 per cent, increase 
the value of /a. Aluminum and silicon act in this way. 

The physical condition of the metal also affects its per- 
meability. Chilling in the mold, when casting, lowers it, as 
does tempering, or hardening the metal by working it. On 
the other hand, annealing increases the permeability. 

A piece of iron or steel, subjected to a small magnetizing 
force, has its permeability increased by increasing the tem- 
perature until a critical temperature is reached, when it falls 
off rapidly to almost unity. For stronger magnetization 
the permeability does not rise so high at the critical tem- 
perature, and does not fall off so sharply after it. The 
value of this critical temperature lies between 650 C. and 
900 C, depending on the test piece. 

24. Reluctance and Permeance. — In the flow of mag- 
netic lines of force the reciprocal of the permeability, -, is 



26 DYNAMO ELECTRIC MACHINERY. 

called the reluctivity. The total reluctance, tending to 
oppose the passage of magnetic lines under the influence 
of a magnetic difference of potential, is directly as the 
length and the reluctivity of the medium and inversely as 
us cross-section. Hence the total magnetic resistance or 

Reluctance = — : — reluctivity. 

cross-section J 

Reluctivity is usually represented byp(=-). Hence for 

a medium of cross-section A square centimeters and length 
/ centimeters, the reluctance 

Permeance is the reciprocal of the reluctance, hence the 
permeance ^ ^ 

It must be remembered that p and fx are not constant for 
any one substance, but depend for their values upon the 
strength of the magnetizing force 3C which is acting upon 
the substance. 

25. Relation Between Magnetomotive Force, Magnetic 
Flux, and Reluctance. — These quantities are related to 
each other the same as are E.M.F., current, and resistance, 

viz 

Magnetomotive Force 

Reluctance 

In this respect electric current and magnetic lines are 
similar. However, while electric circuits, in the main, ex- 
ist in media of zero electric conductivity, and therefore 
permit of accurate calculations, there being no appreciable 
leakage, magnetic circuits must be situated in media which 



MAGNETIC LAWS AND FACTS. 



27 



have permeabilities of at least unity. In the latter case 
much leakage is present, and precise calculations are out of 
the question. In the designing of dynamo electric ma. 
chinery, however, one or more paths of low reluctance are 
presented to the magnetizing force, and these are pro- 
tected by being so shaped that leakage paths offer a com- 
paratively high reluctance. 

26. Hysteresis. — If a piece of iron become magnetized, 
and the magnetizing force be then removed, the iron does 




Fig. 11. 

not become completely demagnetized. A certain magnet- 
izing force in the opposite direction must be used to bring 
it to a neutral state. This phenomenon has been termed 
hysteresis. Because of hysteresis a (B-3C curve taken with 
continuously increasing values of 5C to the maximum and 



2-8 DYNAMO ELECTRIC MACHINERY. 

then with continuously decreasing values of 5C to a negative 
maximum, and so on, will assume the shape shown in Fig. 
ii. The distance O A represents the coercivity, that is, 
the magnetizing force necessary to bring the iron from a 
magnetic to a neutral state. The distance O C represents 
the reteiitivity, that is, the amount of magnetic induction 
left in the iron after the magetizing force has been removed. 
The area inclosed by the curve represents the energy lost 
in carrying the iron through one cycle, i.e., from a maximum 
magnetization to a maximum in the opposite direction and 
back to the orginal condition. 

For suppose the magnetization to be due to a current / 
flowing in a solenoid of ;/ turns. If, in a short interval of 
time dt, a change of d^> be made in the flux which is linked 
with the solenoid, then this change will induce an E.M.F. 
in the solenoid which during the interval of time dt will be 
equal to 

E = — f- volts. 

i oV/ 

During this time work must be performed to maintain this 
current /, and its magnitude is 

nld$ 

Eldt = f , 

io 8 

for Idt represents the quantity of electricity which is trans- 
ferred from one point to another, between which there ex- 
ists a difference of potential E. Now <f> = A(& (§ 22) and 

10 / rc/ 
hence d<\> = Ad®>. Furthermore, nl = (§ 21). Hence 

the work during the time dt is 

Al 
Eldt =—. -.— 3C</(B joules. 

I0'47T 



MAGNETIC LAWS AND FACTS. 29 

Supposing the magnetizing force to vary cyclically, taking 
T seconds to make one cycle, then the work per cycle is 

Al C ~^~ ® waa: 
= — = — / 0C//CB joules. 

I0 7 47Tj (K 



EIT 

- ($> 



mux 



If the number of cycles completed in one second be/, then 
f — — , and the work in joules per second, that is, the power 
in watts, equals 

BI= S*L fttm* ^/volume ft 

IO"47rJ _ ( n io 8 J _ 



'max 



'max ^max 

The integral expression is evidently the area contained by 
the hysteresis loop. 

27. Steinmetz's Law The value of the integral ex- 
pression is dependent upon (B mcKC , upon the retentivity of 
the kind of iron, and upon its coercivity. Steinmetz has 
shown that for all practical purposes the value of the inte- 
gral may be expressed by the empirical formula 

I (* + ®max 

47Tj ' max 

^max 

where rj is a constant depending upon the kind of iron. 
Its value is given in the following table : — 

HYSTERETIC CONSTANTS. 

Best soft iron or steel sheets 0.001 

Good soft iron sheets 0.002 

Ordinary soft iron 0.003 

Soft annealed cast steel 0.008 

Cast steel 0.012 

Cast iron 0.016 

Hard cast steel 0.025 



30 



DYNAMO ELECTRIC MACHINERY. 



The hysteretic constant, if at first small, grows with age. 
Its increase can be hastened by continued heating. The 
increase may amount to 200 per cent. Annealing, while it 
increases the permeability, also increases the hysteretic 
constant, if it be originally very small. 

The magnitude of the hysteretic constant is largely de- 
pendent upon the mechanical structure of the iron. To 
attain the smallest value the iron should not be of homoge- 
neous structure, but should have a greater density in the 
direction perpendicular to the direction of flux. 



ARMATURES. 3 1 



CHAPTER III. 

ARMATURES. 

28. Dynamos Dynamos may be denned as machines 

to convert mechanical energy into electrical energy by 
means of the principle of electromagnetic induction. In 
all commercial machines the mechanical energy is supplied 
in the form of rotation, and the electrical energy is deliv- 
ered either as "direct current" or "alternating current." 
These machines are also frequently called generators. 

29. Principle of the Action of a Dynamo. — If a loop of 
wire be revolved in a magnetic field about an axis perpen- 
dicular to the lines of force, as in Fig. 1 2, then each side 
(but not the ends) of the loop is a conductor moving across 
the lines of a magnetic field, and as such will have an 
E.M.F. induced in it. Since the motion of one conductor 
is up while that of the other is down, the directions of the 
induced E.M.F.'s in the two sides will be opposite to each 
other, and since they are on opposite sides of a loop, the 
pressure will be cumulative ; i. e., instead of neutralizing 
each other, the two pressures will be added to each other. 
If now the two ends of the wire from which the loop is 
made be respectively connected with slip rings, and a cir- 
cuit be completed through contacts sliding on them, a cur- 
rent will flow. When the loop, in its revolution, reaches a 
position (as illustrated in Fig. 1 2) such that the conductor 



32 



DYNAMO ELECTRIC MACHINERY. 



that was previously moving upward begins to move down- 
ward, then the direction of the induced E.M.F. will be 
changed in both sides of the loop, and the direction of the 




Fig. 12. 

current through the circuit will be changed. For each 
complete revolution the current changes direction twice. 
It is an alternating current, and the supposed machine is 
an alternating current dynamo, or simply an alternator. 

30. The Principle of the 
Commutator A commu- 
tator is used on the shaft of 
a machine when it is de- 
sired to get a direct or rec- 
tified current. For the 
single loop in the above 
Flg * I3 * case, the commutator (Fig. 
1 3) would consist of two similar cylindrical parts of metal, 
insulated from each other, and affording sliding contact for 




ARMATURES. 



'33 



two brushes. One end of the wire of the loop is attached 
to one piece of the commutator, and the other to the other. 
The brushes are so placed that at the instant the in- 
duced E.M.F. in the loop changes its direction, the brushes 
slide across from one segment of the commutator to the 
other, and thus the current, while reversed in the loop, is 




Fig. 14. 

left flowing in the same direction in the outside circuit. 
If the loop were wound double before the ends were at- 
tached to the commutator segments, and if the speed of 
revolution and the strength of the magnetic field were both 
maintained constant, twice the E.M.F. would be produced, 
but no more commutator segments would be necessary 
(Fig. 14). 

In the above cases at the instants of commutation ■ there 
would be no E.M.F . produced, and hence the current would 
fall to zero twice every revolution. If two coils were placed 
90 apart, one or the other would always be cutting lines 
of force. Hence at no time could the pressure be zero. 



34 



DYNAMO ELECTRIC MACHINERY. 



To satisfactorily collect current from this arrangement re. 
quires four commutator segments and a system of connec- 
tions similar to that shown 
in Fig. 15. In this case 
the E.M.F. would fluctu- 
ate, but not so badly as in 
the previous case. If we 
increase the number of 
loops, and correspondingly 
increase the number of 
commutator segments, we 
decrease the fluctuation 
of the E.M.F. until it be- 
comes practically constant. In a bipolar machine with 12 
commutator segments the fluctuation is 1.7 per cent of the 
total E.M.F. 




Fig. 15. 



31. The Armature. — In a dynamo, the loops of wire in 
which E.M.F. is induced by movement in a magnetic field, 
together with the iron core that sustains them, with the 
necessary insulation, and with the parts connected imme- 
diately thereto, constitute the armature of a dynamo. The 
conductors in which the 
E.M.F. is generated are 
called the inductors. An 
armature in which both 
sides of the loop of wire 
cut lines of force, as in 
the cases just described, 
is called a Drum Arma- 
ture. A kind of armature less generally used is the Ring 
Armature, illustrated diagrammatically in Fig. 16. Here 




ARMATURES. 



35 



the lines of force emanating from the N. pole of the field 
magnets flow through the iron core of the ring, and very 
few across the air space inside the ring. Hence when 
wires are wound on the ring, and the whole is revolved 
about an axis perpendicular to the plane of the ring, only 
the wires on the outside face of the ring cut lines of force, 
those on the inside serving only to complete the electrical 
circuit. So a smaller portion of the wire on a ring arma- 
ture is in action than on a drum armature. 

A drum armature of large diameter and of short length 
in the axial direction has more wire exposed on its ends 
than on its periphery. The pole pieces are sometimes 
placed at the ends, and the armature is then called a Disk 
Armature. This type is seldom used in this country. 

32. The Field Magnets. — Almost all dynamos have 
their magnetic fields produced by electro-magnets. These 




Fig. 17. 



are called the field magnets. In small machines these are 
usually bipolar, i.e., having one N. and one S. pole, with 



36 DYNAMO ELECTRIC MACHINERY. 

the armature revolving between. In large machines it is 
usual to use multipolar field magnets, in which any even 
number of poles alternately N. and S. are arranged in a 
circle with their faces concentric with the armature. 

Bipolar machines are made in many forms, a few of 
which are shown in Fig. 17. 

The magnetizing coils may be on both legs of the mag- 
net, on one leg, or on the yoke which connects the two legs. 
In the double horse-shoe type there are four windings, one 
on each of the four legs. Such a field is sometimes said 
to be of the consequent pole type. 

33. Capacity of a Dynamo. — By § 13, in a bipolar 
machine the average pressure between brushes equals the 
product of the number of lines cut into the number of in- 
ductors cutting them, divided by the time in seconds of 
one revolution. Since each line is cut twice in one revolu- 
tion by each conductor, the formula for the E.M.F. pro- 
duced by the machine is 

60 io 8 

where V is the number of revolutions per minute, <£ the 
total flux through the loops, and 5 the number of inductors. 
In drum armatures 5 == twice the number of loops ; in ring 
armatures 5 = the number of loops. 

The capacity of a machine is measured by the watts it 
can send out, hence the capacity varies as EI It is seen 
from the foregoing formula that the Eoi any machine may 
be increased by increasing either V, <£, or S. 

The value of Fis limited, (1) by considerations of me- 
chanical safety and economy, and (2) by the desirability, in 
the case of a dynamo, of directly connecting it to the steam 



ARMATURES. 37 

engine or other prime mover, and in the case of a motor 
the connection of it to the machine it operates. The speed 
of small machines is greater than that of larger ones ; but 
the peripheral velocity, that is, the velocity of a point on 
the exterior of the armature, for all sizes, lies between 25 
and 100 feet per second on belt-driven machines, and be- 
tween 25 and 50 feet per second on direct connected 
machines. On large (say 2,000 k.w.) multipolar machines, 
having great diameter of armature, these values are often 
exceeded. 

The value of <f> depends upon the size of the machine, 
and the permeability of the metal of its frame. To get a 
large and economical (B the metal parts of the field magnets 
are designed to have a very low magnetic reluctance. The 
air-gap between the pole pieces and the armature, and the 
space occupied by the revolving inductors, are each made 
small. The armature inductors are wound upon an iron 
core of low magnetic reluctance. These cores are fre- 
quently slotted and the windings laid in the slots. Besides 
reducing, to a certain extent, the magnetic reluctance by 
this construction, a good mechanical means is furnished 
for driving and protecting the inductors. Wires wound on 
the exterior of a plain cylinder, or smooth core, under the 
influence of high speeds and the "magnetic drag" which 
they experience have a serious tendency to rub one an- 
other, and chafe the insulation to its final destruction. 
The armatures having slotted cores, which are also called 
toothed core armatures, are to be recommended for gene- 
rators that will be obliged to work under wide variations of 
load. They cost more to build than smooth-core arma- 
tures. 

The numbers of inductors vS on an armature can be in- 



38 



DYNAMO ELECTRIC MACHINERY. 



creased by decreasing the size of the wire. Sufficient 
cross-section must, however, be left in the inductors to 
carry the maximum current of the machine without causing 
a heating of the armature to such a point as to endanger 
the insulation. Good practice calls for from 400 to 800 
circular mils cross-section of armature conductor per am- 
pere. The smaller values are for intermittently acting 
machines — elevator motors for example. The larger 
values are for machines that run continuously, such as 
central-station generators. 

34. Eddy or Foucault Currents in Armature Cores. — 

It is evident that an imaginary axial lamina of the iron core 
of an armature is a conductor moving in a field, and there- 
fore has in it an induced E.M.F. Since this lamina in it- 




Fig. 18. 

self forms a closed circuit, currents, called Foucault or eddy 
currents, will flow in it, Fig. 18, and their energy will 
appear in the form of heat, which will produce an undue 
elevation of temperature of the armature. To avoid this 
the iron of the core is laminated at right angles to the axis 
of revolution, and the laminae are insulated from one 



ARMATURES. 39 

another. The heating due to eddy currents is proportional 
to the square of the thickness of the disks or laminae. 
Commercial and mechanical reasons limit the decrease of 
thickness. In good practice the thickness of armature 
disks varies from .01" to .06. " 

For insulation between the disks reliance is usually 
placed on the iron oxid that forms on them during their 
manufacture. Generally every six disks or so a further 
insulation is interposed by the use of shellac, japan, or 
paper. Milling slots in laminated armature cores after set- 
ting up causes burrs. These bridge the insulation between 
the disks, and militate against the advantages sought after 
by lamination. For small armatures the disks are punched 
whole from sheet-iron, with the teeth and holes for the 
shaft. These punchings are assembled on the shaft, and 
held in place by brass collars set down on either side of the 
pile by nuts on the shaft or by similar devices. In large 
machines, parts or segments of the whole periphery are 
punched separately, and these are assembled with joints 
staggered. These large laminae are not directly attached 
to the shaft, but are mounted upon a spider, which in turn 
is connected with the shaft. A complete spider and core 
is shown in Fig. 19. 

In large armatures it is usual to make ducts or venti- 
lating passages in the core by occasionally separating the 
disks by the interposition of blocks of insulating material. 
Such ventilation carries off the heat, and lessens the rise of 
temperature of the armature when in operation. 

35. Rating of Machines. — The American Institute of 
Electrical Engineers recommends that all electrical and 
mechanical power be expressed, unless otherwise specified, 



40 



DYNAMO ELECTRIC MACHINERY. 




Fig. 19. 



ARMATURES. 41 

in kilowatts ; that the full-load current of an electric gene- 
rator be that current which, with the rated full-load volt- 
age, gives the rated kilowatts ; that all guaranties on heat- 
ing, regulation, and sparking should apply to the rated 
load, except where expressly specified otherwise ; that 
direct current generators should be able to stand an over- 
load of 25 per cent for one-half hour without an increase 
of temperature elevation exceeding 15 C. above that 
specified for full load ; and that direct current motors 
should, in addition, be able to stand an overload of 50 per 
cent for one minute. 

Concerning the normal permissible elevation of tempera- 
ture the following statements are taken from articles 25 to 
31 of the Institute's Standardization Report : — 

" Under regular service conditions, the temperature of 
electrical machinery should never be allowed to remain at 
a point at which permanent deterioration of its insulating 
material takes place. 

" The rise of temperature should be referred to the stan- 
dard conditions of a room temperature of 25 C, a baro- 
metric pressure of 760 mm. and normal conditions of 
ventilation ; that is, the apparatus under test should neither 
be exposed to draught nor inclosed, except where ex- 
pressly specified. 

" If the room temperature during the test differs from 
2 5 C, the observed rise of temperature should be cor- 
rected by i per cent for each degree C. Thus, with a 
room temperature of 35 C, the observed rise of tem- 
perature has to be decreased by 5 per cent, and with 
a room temperature of 15 C, the observed rise of tem- 
perature has to the increased by 5 per cent. The 
thermometer indicating the room temperature should 



42 DYNAMO ELECTRIC MACHINERY. 

be screened from thermal radiation emitted by heated 
bodies, or from draughts of air. When it is impracti- 
cable to secure normal conditions of ventilation on ac- 
count of an adjacent engine, or other sources of heat, the 
thermometer for measuring the air temperature should be 
placed so as fairly to indicate the temperature which the 
machine would have if it were idle, in order that the rise of 
temperature determined shall be that caused by the opera- 
tion of the machine. 

" The temperature should be measured after a run of 
sufficient duration to reach practical constancy. This is 
visually from 6 to 1 8 hours, according to the size and con- 
struction of the apparatus. It is permissible, however, to 
shorten the time of the test by running a lesser time on an 
overload in current and voltage, then reducing the load to 
normal, and maintaining it thus until the temperature has 
become constant. 

" In apparatus intended for intermittent service, as rail- 
way motors, starting rheostats, etc., the rise of temperature 
should be measured after a shorter time, depending upon 
the nature of the service, and should be specified. 

" In apparatus built for conditions of limited space, as 
railway motors, a higher rise of temperature must be 
allowed. 

" In electrical conductors, the rise of temperature should 
be determined by their increase of lesistance. For this 
purpose the resistance may be measured either by galva- 
nometer test or by drop-of -potential method. A temperature 
coefficient of 0.4 per cent per degree C. may be assumed 
for copper. Temperature elevations measured in this way 
are usually in excess of temperature elevations measured 
by thermometers. 



ARMATURES. 



43 



" It is recommended that the following maximum values 
of temperature elevation should not be exceeded : — 

COMMUTATING MACHINES. 

Field and armature by resistance, 50 C. 

Commutator and brushes by thermometer, 55 C. 

Bearings and other parts of machine, by thermometer, 40 C. 



" Where a thermometer, applied to a coil or winding, in- 
dicates a higher temperature elevation than that shown by 
resistance measurement, the thermometer indication should 
be accepted. In using the thermometer, care should be 
taken so to protect its bulb as to prevent radiation from it, 
and, at the same time, not to interfere seriously with the 
normal radiation from the part to which it is applied. 

" In the case of apparatus intended for intermittent ser- 
vice, the temperature elevation, which is attained at the end 

of the period corresponding to 
the term of full load, should 
not exceed 50 C. by resistance 
in electric circuits. In the case 
of railway, crane, and elevator 
motors, the conditions of ser- 
vice are necessarily so varied 
that no specific period corre- 
sponding to the full-load term 
can be stated." 
The manner in which temper- 
ature elevation is affected by size of load and duration of 
full load is shown in Figs. 20 and 21. The temperature 
of stationary surfaces rises about 8o° when radiating one 
watt per square inch. The rise is but 15 to 20 when the 



300 


□ 


1 1 1 1 1 1 1 1 1 1 






-2- 

-Z- 




RISE IN TEMPERATURE CURVE 

300 K. W. SIZE 280 

DIRECT DYNAMO 

RUN LONG ENOUGH ATTACH LOATJ TO 

ATTAIN A CONSTANT TEMPERATURE 

CROCKER-WHEELER ELECTRIC CO., 

AMPERE, N. J. 




- 






/ 


200 




/ 































































100 


= 




























^ 






























JO. 



























































°= 




























-1 
































(. 


OA 


IN 


TE 


RM 


O 


■ FL 




LO/ 


D 


4 


22 



1 

Fig. 20. 



44 



DYNAMO ELECTRIC MACHINERY 



surface is rotating at 3,000 feet per minute- in such a 
manner as the surface of an armature rotates, and amounts 





■C-- 


















































i J-- 




































"^ 




































i 


1 






















z 
















TEMPERATURE CURVE 

300 K. W. SIZE 280 

DIRECT DYNAMO 






















































CROCKEF 


-WHEELER ELECTRIC CO. , 
Ampere, N. J. 






-J 

-2- 
< 




















































1 






































































































































































































































TIME IN HO^URS 














421 



3 4 5 
Fig. ax. 



8 9 10 



to but IO° to 12 



at a speed of 6,500 feet per minute. 
Within limits, the ratio of rise of temperature to radiation 
per unit surface is linear. 

36. Definitions Concerning Armature Windings. — In 
some dynamos the inductors and commutator segments are 
not all electrically connected with each other. In such 
cases the winding is called an open-coil winding. This 
definition must not be made to include the double or mul- 
tiple windings to be described later, where two or more 
closed-coil windings on the same core are not electrically 
connected to one another. Fig. 22 shows a primitive open- 
coil winding. In this type only those inductors on whose 
commutator bars the brushes may for the moment be rest- 
ing are in series with the external circuit. All the other 
inductors are cut out and idle. 

Open-coil windings are used chiefly on arc-lighting dyna- 
mos, and will be further discussed in a following chap- 



ARMATURES. 



45 



ter devoted to such machines. Closed-coil windings are 
much more generally used. In this case all the inductors 
are engaged all the time, save when short circuited at com- 
mutation, in adding E.M.F. to the circuit. Although there 
are many kinds of closed-coil windings, they are all alike 
in that the inductors form one or more endless circuits 
completely around the armature core. 

Before showing some of the many types of closed-coil 
winding it will be well to define some of the terms used. 




/WWWWWM 

Fig. 22. 



By inductor is meant that part of the winding conductor 
which lies on the face of the armature that sweeps past 
the pole pieces, and is that part of the conductor in which 
E.M.F. is induced. In the following descriptions when 
one inductor is mentioned there may be in reality a num- 
ber of wires ; and, again, a loop said to be formed by two 
inductors may be a loop of many turns, but the connec- 
tions and placing would be the same as if actually there 



46 DYNAMO ELECTRIC MACHINERY. 

were only one inductor. It simplifies the diagrams to 
treat the subject in this manner. 

That part of an armature winding which is electrically 
connected directly between two consecutive commutator 
segments is called a coil. 

A two-circuit winding is one in which the current, on 
entering the armature at one brush, finds two paths by 
which it reaches the other brush. Since a closed-coil 
winding is endless, there must invariably be two paths 
when two brushes are used. 

A four-circuit or multi-circuit winding is one in which 
the current finds four or more paths through the arma- 
ture. There are at least two circuits for every pair of 
brushes used in collecting the current, unless the commu- 
tator bars are cross-connected, as in Fig. 25. 

If a winding is so arranged that one commutator bar 
under a brush carries all the current from one side of the 
armature to that brush, then the winding is said to be 
single. If, however, the windings are so arranged that 
two or more bars convey this current to the brush at once, 
or if the current is commutated at two or more points on 
the contact surface of the brush, then the winding is said 
to be double or multiple. Triple and quadruple windings 
are not infrequent on machines which carry very heavy 
currents. 

A singly-re-entrant winding is one in which, by successive 
angular advances, all the coils have been laid when an 
advance of 360 has been made. To be doubly-re-entrant 
wound the angular advance between successive coils, in the 
order of their winding, is doubled ; and the whole winding 
is not complete until the armature has been gone around, 
angularly, twice, i.e., through an advance of 720 . On the 



ARMATURES. 



47 



second time around the coils fill up the interstices left by 
the doubled pitch of the first round. Triply and quadruply 
re-entrant windings are used. In these the circuit passes 
around the armature three or four times. Any closed-coil 
winding, single or multiple, may be singly or multiply re- 
entrant, the re-entrancy being reckoned as great as that of 
any single winding on the armature. 

The two principal types of closed-coil armatures are 
the gramme or ring armature, and the drum. 

37. Ring-Armature Windings. — As the name implies, 
the ring-armature core consists of an annular ring around 
which the armature conductors are wound in a continuous 
spiral, or two or more separate but interleaved spirals in 
multiple windings. These are tapped off at equal inter- 
vals to the commutator bars. In ring armatures there is 
but one inductor per loop of wire, the return being on the 
inside of the ring where there is no magnetic flux. This 
winding, though less generally used than the drum winding 
is simpler and much more easily illustrated, and will be 
treated first. 




mm 




Fig. 24. 

Fig. 23 shows the simplest of all dynamo armature 
windings. It is a bipolar, singly-re-entrant, two-circuit, 
single winding. 

Fig. 24 shows a four-pole, four-circuit, singly-re-entrant, 



43 



DYNAMO ELECTRIC MACHINERY. 



single winding. The number of coils should be a multiple 
of the number of poles to electrically preserve a balance in 
the four branches or circuits. 

Fig. 25 is the same as Fig. 24 save that the commutator 
bars are cross-connected. The current that would flow 
out of two brushes in the previous case, now flows out of 
one brush. This form is seldom used, since it reduces by 
half the brush contact surface, and thus doubles the heat 




Fig. 25. 




Fig. 26. 



loss in the transition of the current from the commutator 
to the brush. 

Fig. 26 shows a four-pole four-circuit singly-re-entrant, 
single winding, where only half as many bars are used as 
there are coils. A disadvantage is that coils of considerable 
difference of pressure are adjacent, thus increasing the 
difficulty of properly insulating them. Ordinarily, if it be 
desired to halve the number of bars, it is better to unite 
two adjacent coils in series, and treat them as one. But 
if the magnetic distribution be uniform, this method of 
connecting two coils that are in different parts of the field 



ARMATURES. 



49 



in series averages up the inequalities and facilitates spark- 
less commutation. 

Fig. 27 shows a bipolar, two-circuit, singly-re-entrant, 
The advantages of the double winding 
current is commutated 



double winding. 

are : the 

at two points of the bearing-sur- 




Fig. 27. 



face of the brush, and therefore is 
only half as heavy at any one point 
as when only a single winding is 
used ; and the successive bars of 
one winding are separated by the 
width of one bar plus two insula- 
tions, thus making the short cir- 
cuiting of a coil by dirt, arc, or 
injury very unlikely. 

In multipolar windings a distinc- 
tion is made between the " short- 
connection " and the " long-connection " types. In the 
short-connection type coils in adjacent fields are connected 

in series, while in the long-con- 
nection type coils twice as far 
apart are connected together. 

Fig. 28 shows a long-connec- 
tion, two-circuit, four-pole single 
winding. Here only slight dif- 
ferences of potential exist be- 
tween contiguous coils. 

Fig. 29 represents a ten-pole, 
long-connection, two-circuit, sin- 
gle winding. In these long-con- 
nection types, which are all more or less highly re-entrant, 
small mention is made of the re-entrancy. Strictly accord- 




5o 



DYNAMO ELECTRIC MACHINERY. 



ing to definition, the winding in Fig. 29 is re-entrant nine 

times. 

Fig. 30 is a four- 
pole, short -connection, 
two-circuit, single wind- 
ing. Besides the com- 
plication of the wind- 
ings, this form as well 
as all other short-con- 
nection windings, is 
open to the objection 
that the contiguous 
coils have, periodically, 
the full E.M.F. of the 
machine between them, 

making heavy insulation necessary. 

Fig. 31 gives a four-pole, two-circuit double-wound 




Fig. 29. 




Fig. 30. 



armature, and Fig. 32 gives a similar winding for a six- 
pole machine. 



ARMATURES. 



51 



38. Drum Armature Windings. — Windings for drum 
armatures are more varied and more complex than those 
for ring armatures, and are much harder to portray dia- 
grammatically. But few will be shown. 

The most simple of these windings is shown in Fig. 33. 
The diagram shows the drum and inductors in section, 
with the connections of the commutator end in full lines 
and those of the back (pulley) end in dotted lines. Those 
inductors marked with a + are supposed to carry a current 
in the direction from the observer into the paper, and 




Fig. 32. 



Fig. 33- 



those with a point are supposed to carry a current from 
the page to the observer. Those not marked are parts of 
coils short-circuited by the brushes. This winding was 
devised by von Hefner-Alteneck, and may be used on any 
bipolar armature having half as many commutator bars as 
slots, or, if it be smooth core, as many bars as coils. If n 
be the number of bars and 2n the number of slots, then 
the wire is started at bar 1, passed back through slot 1, 
across the pulley end to slot n (or sometimes 11 ± 2, in the 



52 



DYNAMO ELECTRIC MACHINERY. 



figure 11 = 8). It is then brought forward through slot n, 
and attached to bar 2. From bar 2 it passes back through 
slot 3, across the back end and forward through slot 11 + 2 
and connects to bar 3. Thus passing back through the 
odd-numbered slots and forward through the even-num- 
bered slots, n coils can be made to fill the 211 slots and 
each can be attached to its own commutator bars. 

Fig. 34 is very similar to the last, save that the wires 
are laid two layers deep, thus allowing the conductor that 

passed through slot 1 to return 
through slot ;/ + 1 which is 
diametrically opposite. Both 
this winding and the last are 
classed as two-circuit single 
windings. 

In a bipolar machine a chord- 
wound drum armature is one in 
which the two inductors of one 
loop are appreciably less than 
180 apart, so that the wire at 
Fig - 34> the back end is a chord rather 

than a diameter of the circle of the drum. The advan- 
tages of this winding are that, on a given drum, it de- 
creases the total length of wire necessary to give a definite 
number of inductors, and that it reduces the bunching and 
overlapping of the wires at the pulley end of the drum. 
The disadvantage is that it is impossible to secure a per- 
fectly electrically balanced winding by this method. This 
objection does not hold in the case of multipolar gene- 
rators, hence all multipolar drums are chord wound. 

In the following figures the numbered radial lines will 
represent armature inductors, the lines inside of them will 




ARMATURES. 



53 



represent their connections to the commutator segments, 
and the lines outside of them will represent the cross 
connections between inductors at the pulley end. The 
brushes are placed in- 
side the commutator 
for convenience and 
clearness. 

Fig. 35 represents a 
six-circuit, single wind- 
ing with 80 inductors 
and 40 segments. In 
practice the inductors, 
instead of all lying be- 
side each other, would 
probably be wound one 
on top of another in 
one slot. 

Fig. 36 shows a rather simple single winding. Although 

it is four pole it is but 
two circuit, in which it 
resembles Fig. 37, which 
is, however, a triple wind- 
ing. 

Fig. 38 gives a six- 
pole, two circuit, double 
winding. 

In winding armatures, 
double or triple cotton in- 
sulated copper wire is 
generally used. Care 
must be taken to well in- 
sulate the wires, both from each other and from the core. 





54 



DYNAMO ELECTRIC MACHINERY. 




Many of these styles of winding create very complex 

masses of wire on the 
ends of the drum, and 
great care must be ex- 
ercised both in regard to 
insulating and fastening 
at these points, so that 
the movement of the 
wires under the influence 
of the " magnetic drag" 
may not chafe the insu- 
lation and short-circuit 
s the conductors. Mica is 

Fig- 37. the best insulator, and is 

used where flat sheets are needed ; but its great cost, and 
the difficulty of manipulating it, result in the extensive use 
of canvas, oiled paper, rubber tape, vulcanized fiber, and 
many patented manufac- 
tured insulators. Much 
reliance is placed upon 
the liberal use of japan 
and shellac, especially in 
conjunction with canvas. 
Where very large 
wires are used on the 
surface of an armature, 
eddy currents are set up 
in them by reason of one 
side of the wire being 
in a stronger field than ***• 38, 

the other. To avoid this a number of smaller insulated 
wires are wound in parallel, to take the place of the larger 




ARMATURES. 55 

one, or what is more economical of space, thin copper bars 
set edgewise take the place of the round wire. 

In the winding of multipolar armatures it is possible to 
use formed coils, which are wound on a separate collapsible 
forming block, and are afterward applied to the core. This 
method is advantageous in that better insulation can be as- 
sured, and damaged or burned-out coils can be replaced 
without disturbing all of the windings. Fig. 39 shows a 
General Electric Company's formed coil, and Fig. 40 some 
of the Crocker Wheeler coils. 




Fig. 39. 

All armatures, whether wound with wire, or formed coils, 
or shaped conductors, must be banded around to prevent 
dislodgement of the conductors under influence of cen- 
trifugal action. The wire used for this purpose is gener- 
ally of hard-drawn brass or of phospher bronze, and on 
railway motors of steel. It is wound over insulating strips 
forming a band of several turns. The completed turns 
are often sweated together with solder. 

Many manufacturers punch a small recess in each side 
of the teeth near the face. A strip of maple wood is fitted 



56 



DYNAMO ELECTRIC MACHINERY, 




ARMATURES. 



57 





Fig. 4 i. 



Fig. 42. 



into the recesses, and acts like a cover to the slot, firmly 
holding the windings in place, and presenting a neat ap- 
pearance. 

Figs. 41, 42, 43 show respectively a core, a partially 
wound, and a completed General Electric Company's arma- 
ture. Figs. 44 and 45 show small Westinghouse types. 



58 



DYNAMO ELECTRIC MACHINERY. 



Fig. 43- 





Fig. 45- 



ARMATURES. 59 

39. Commutators. — The segments or bars of a commu- 
tator are always of drop-forged, or hard-drawn copper. 
The insulation between them is always of mica. There 
are various grades of mica ; and for insulating purposes the 
amber-colored mica, which must be free from iron, is to be 
preferred. Besides being a good insulator, amber mica has 
the additional advantage that it wears at the same rate as 
copper ; thus after long use it leaves neither elevations nor 
depressions on the commutator surface. 

In fastening the bars considerable ingenuity is displayed ; 
for they must not displace themselves with reference to 
the windings, neither must one bar lift so as to be above 
the level of its neighbors. If the latter occurs, then, when 
the bar comes under a brush, it will lift it ; and as the 
high spot moves out from under the brush the contact is 
broken until the spring can reseat the brush. This causes 
excessive wear and destructive sparking. 

After a commutator has been for a time in use, it becomes 
grooved and pitted, a condition which causes further spark- 
ing and wear, and the commutator must be turned down 
again to a true surface. The design of a commutator 
should allow of good operation after it has been subjected 
to this treatment. 

Mechanical friction and the electrical losses that accom- 
pany commutation will raise the temperature of the com- 
mutator about 5 C. above that of the armature. To 
secure successful operation a commutator must be de- 
signed with a sufficient number of bars, so that the differ- 
ence of potential between two adjacent bars shall not 
exceed 10 volts. This would mean that a 100-volt bi- 
polar machine should have at least 20 bars. The potential 
between the brushes or around half the commutator is 100 



60 DYNAMO ELECTRIC MACHINERY. 

volts, hence half the commutator must have 10 bars. 
There is no general rule for the length of a commutator 
bar, but one may roughly say that there should be at least 
one inch per ioo amperes. 

Commutators should be designed so as to expose a suffi- 
cient area to radiate the heat which is communicated to 
them. Except in the case of some special commutators, 
which are supplied with cooling devices, at a peripheral 
speed of 2,500 feet per minute, the radiation of one watt 
per square inch of peripheral radiating surface will re- 
sult in a rise of temperature of 20 C. The permissible 
rise of 55 C, therefore, allows a radiation of 2.75 watts 
per square inch. The heat to be radiated is due to the fol- 
lowing causes : — 

a. Friction between the brushes and the commutator 

bars. This is equal to ( ) times the product of 

V 33'°°° J 
the following quantities : The radius of the commutator in 

feet, the speed in revolutions per minute, the coefficient 
of friction between the brushes and the commutator (0.3 
for carbon brushes and 0.25 for copper brushes), and the 
sum of the pressures of all the brushes upon the commuta- 
tor. This latter should amount to 1.25 lbs. per square inch 
of rubbing surface. Copper brushes permit 200 amperes 
per square inch of rubbing surface, and carbon brushes 
40 amperes. 

b. The contact resistance between the brushes and the 
commutator. As there is always a drop of about one volt 
at each point of contact, and as there is a drop at both the 
positive and negative terminals, the watts represented by 
these contact resistances are numerically equal to twice the 
current of the machine. 



ARMATURES. 



6l 



c. The energy represented in the sparking at the brushes 
and the heat due to waste currents in the short-circuited 
segments. These two losses cannot be accurately calcu- 
lated, but may be estimated as equal to about 6 per cent of 
the total commutator loss. 



Outer rnica cones 



Inner mica cones 



Clamping* ^'^Q 



•Snell 




fflioa collar- under- segment s 



Cap 



Fig. 46. 



Fig. 46 gives a broken-away view of a General Electric 
commutator, showing the methods of attachment and insu- 
lation. 



40. Collecting Devices. — These consist of the brushes, 
the brush holders, and the rockers. 

Brushes for high potential machines are of carbon. Car- 
bon against copper causes less wear than copper against cop- 
per, and further, the greater resistance of a carbon brush 
results in less sparking when it bridges two commutator bars 
than would the lower resistance of a copper brush. Com- 
bination brushes of carbon and copper are sometimes used. 
Carbon brushes are set at an angle generally, though some 
makers set them radially ; and in motors that must be re- 




62 DYNAMO ELECTRIC MACHINERY. 

versed, as is the case with railroad and elevator motors, 
they are invariably set radially. A surface contact of one 
square inch per 40 amperes is usual for carbon brushes. 

On low-potential machines copper brushes, set at an angle 
of 45 with the tangent to the commutator surface at the 
point of contact, are invariably used. This is because there 
is less natural tendency to spark on low voltages, and be- 
cause the resistance of carbon 
brushes would be too great a 
fraction of the whole resistance of 
the circuit, and cause a wasteful 
drop of potential. Copper brushes 
FigT^. must have their ends filed to give 

sufficient surface contact, and this 
is generally done with the aid of a. jig, illustrated in Fig. 47. 
The abrasion of carbon brushes is accomplished by means 
of glasspaper. 

Brush holders should permit of a low-resistance contact 
between the brush and the leads, they should provide ad- 
justment as to position and tension of the brushes, and 
they should be arranged so that none of the springs shall 
get hot and lose temper while in performance of its duties. 
The tension on carbon brushes varies from 1 to 10 lbs. per 
square inch of contact surface. The lower limit is to be 
found in large central station generators, and the higher 
limit in small machines and in motors which are subjected 
to frequent and sudden strains, as railway motors. The 
coefficient of friction between brush carbon and copper 
varies from 0.28 to 0.32. 

Figs. 48 and 49 plainly show a Crocker Wheeler rigging 
with parallel-motion brush holders. Fig. 50 shows a form 
of General Electric holder. 



ARMATURES. 




Fig. 4 8. 

1A CLAMPING SCREW 




ADJUSTING 
SCREW 



HARD ROLLED COPPER LEAVES 



1 2 3 

Scale of Inches 



64 DYNAMO ELECTRIC MACHINERY. 

Rockers are rings or attachments carrying the brush 
holders, and they are mounted concentric with the com- 
mutator. They are made to give all the brushes of the 
machine, or sometimes all the positive brushes or all the 
negative brushes at once, a motion around the axis, thus 
adjusting all brushes by one movement. Fig. 5 1 shows 
such a rocker. 

41. Shafts, Bearings, and Oilers. — Since armature 
shafts generally have high speeds, and almost always are 
subject to sudden large variations of load, the shafts, the 




Fig. 50. 

bearings, and the oiling facilities must be well designed. 
Wiener gives the following approximate diameters of steel 
shafts for drum armatures : — 

For 100 watts I inch, 

For 1,000 watts 2 inches, 

For 10,000 watts 4* inches, 

all to be turned down at the bearings. 

It is necessary that the bearing-boxes be exactly in line, 
and a form of self -alignment bearing is frequently used. If 
undue wear in the bearings occur, the armature is apt to 



ARMATURES. 



65 



sag till it strikes a pole piece, which will damage the arma- 
ture. Many machines use ordinary oil cups to secure 
lubrication, while others make use of some device, as is 
shown in Fig. 52. The shaft revolves in a cylindrical brass 
with a spherical enlargement at its middle which rests upon 




Fig. 51. 

a corresponding spherical bed of Babbit metal. This se- 
cures self-alignment. Two slots are cut radially in the 
brass, and allow two rings to rest upon the shaft. These 
rings are also of brass, and have an inside diameter slightly 
larger than the outside diameter of the brass cylinder. 



66 



DYNAMO ELECTRIC MACHINERY. 



The pillow block is hollowed away under these rings, the 
hollows serving as receptacles for the storage of oil. As the 




Fig. 52. 



shaft revolves, the rings also revolve at such a rate as to 
carry a steady stream of oil up into the slots, thereby 
lubricating the bearing. 



FIELD MAGNETS. 



6 7 



CHAPTER IV. 



FIELD MAGNETS. 



42. Parts of Field Magnets. — The parts of a dynamo, 
exclusive of the armature, which make up the magnetic 
circuit, belong to the field magnets. Fig. 53 shows a con- 
ventional bipolar horse-shoe type with the parts plainly 
marked. The field cores are the iron centers in the mag- 
netizing coils. The yoke connects the cores together at 
one end while the other ends terminate in the pole pieces, 





Fig. 54. 

one being a north magnetic pole, the other a south. The 
side of the pole piece embracing the armature is styled the 
pole face, and the latter's projecting edges are fittingly 
called the horns. Some dynamos have the magnetizing 
coils on the yoke, thus making the latter serve also as 
core. In different types different numbers of pieces pre- 
vail, thus all the parts (save the coils) might be cast in 
one piece or each might be made separately. 



68 DYNAMO ELECTRIC MACHINERY. 

In multipolar machines the designation of the parts is 
somewhat different than in the case of bipolar machines. 
The particular designation often depends upon the manu- 
facturer. Fig. 54 gives the designation used by the 
Crocker Wheeler Company. 

43. Magnetic Material. — The materials used for field 
magnetic circuits are three, — cast iron, wrought iron, and 
cast steel. The selection of material for a given machine 
is governed by considerations of (a) weight, (b) first cost, 
(c) economy and satisfactory regulation when in operation. 

• Cast iron has the great advantage of cheapness ; but it is 
poor magnetically, hence more weight and bulk must be 
employed to perform the same service as the magnetically 
superior wrought iron. It costs more in copper to magne- 
tize a cast-iron core, because more turns will be required, 
and each turn will be longer than if the core were of better 
material. 

Wrought iron is the best magnetic material available. 
It is used either in forgings, or in the form of plates 
punched from the sheet. In either form it is expensive ; but 
since less weight in a given machine is necessitated when 
this metal is used, it is often chosen where portability 
is required, as in the case of the marine dynamos, electric 
railroad motors, and particularly motors for automobiles. 

Cast steel is intermediate between cast iron and wrought 
iron, both in cost and in magnetic properties, and is much 
employed in good practice. The use of different metals in 
different parts of the frame is very general. For instance, 
a cast-iron yoke is used with cast-steel cores and pole 
pieces, or a cast-iron or steel yoke is used with wrought- 
iron cores and pole pieces. 



FIELD MAGNETS. 69 

44. Shape of Field Magnets. — There is a great vari- 
ety of shapes of field magnets. Formerly each manufac- 
turer had a type peculiarly his own, and this led to many 
forms, some of little merit. These freak types are now 
disappearing, and a few general types are adopted more or 
less by all makers. In all forms, however, the polar span, 
or part of the armature circle that is covered by pole faces, 
is from 65 per cent to 75 per cent, or from 234 to 270 . 
In general a small number of poles in the field magnets re- 
quires less copper in the exciting coil than does a larger 
number, and also the fields can be excited more economi- 
cally. But in large bipolar machines successful operation 
under varying loads requires a large air gap between the 
pole face and the armature. This increases the magnetic 
reluctance and the energy necessary for excitation. Multi- 
polar machines do not require so large an air gap. Further- 
more, increasing the number of poles gives the mechanical 
advantage of allowing a lower armature speed without low- 
ering the potential of the output. Multipolar machines 
will run cooler than bipolars of the same economy of 
operation. 

Speaking generally, though it is by no means a rule, 
bipolar fields are used up to about 10 k.w., four-pole fields 
from 10 k.w. to 100 k.w., six-pole fields from 100 k.w. to 
300 k.w., and beyond that point eight or more poles are 
generally used. 

45. Methods of Excitation of Fields. — Dynamos are 
classified according to the five methods of exciting the 
fields of the machine. They are : — the Magneto, the 
Separately Excited, the Shunt Wound, the Series Wound, 
and the Compound Wound. 



7o 



DYNAMO ELECTRIC MACHINERY. 



The magneto generator, Fig. 55, is one in which the 
field is a permanent steel magnet, generally of horse-shoe 
type. 

The separately excited dynamo, Fig. 56, has, as its 





MAGNETO DVNAMO 
Fig- 55- 



SEPARATELY EXCITED DYNAMO 

Fig. 56. 



name implies, its field coils traversed by a current other 
than that produced by the machine. Alternating current 
machines are nearly always of this type. 

The shunt-wound machine, Fig. 57, has a large number 
of turns of fine wire wound on its core, and the ends are 



/- 




\ 
















(fe 


D 













s~ 




' 

1 




r 1 


% 



8HUNT WOUND 
DYNAMO 



SERIES WOUND 
DYNAMO 

Fig. 58. 



Fig. 57- 

connected to the terminals of the machine, thus being in 
shunt with the outside circuit. The ampere turns requisite 
for excitation are obtained by passing a small number of 
amperes through a large number of turns. 

The series-wound generator, Fig. 58, has all the cur- 



FIELD MAGNETS. 



71 



rent that is produced by the armature passed through 
large conductors wound with fewer turns around the cores. 
The exciting coils are then in series with the external cir- 
cuit. The ampere turns required for excitation are ob- 
tained by passing a large current through a small number 
of turns. 

The compound machine, Fig. 59, is one in which there 
are both shunt and series coils on the field magnets. This 
method of winding is used for purposes of regulation under 
varying loads, as will be explained later. Compound wind- 
ings are of two classes, the long shunt and the short shunt. 
In the former, the current used in the shunt windings is 



w 





f 




\ 










<— 












m 


J 











COMPOUND WOUND 
DYNAMO LONG SHUNT 

Fig. 59. 



COMPOUND WOUNO 
DYNAMO SHORT 6HUNT 

Fig. 60. 



also passed through the field windings along with the main 
current. In the latter, the current from the shunt coils 
passes directly back to the armature, avoiding the series 
turns. Figs. 59 and 60 clearly show the two methods. 
The short shunt is generally preferred. 



46. Field Coils. — The coils of a dynamo must, without 
undue elevation of temperature, supply sufficient ampere 
turns to give the required excitation. This temperature 
rise will not be excessive when about 0.35 watt is radiated 
per square inch of outer surface of the coil. If no account 
be taken of the ends of the pole and coil, 0.6 watt may be 



72 



DYNAMO ELECTRIC MACHINERY. 



allowed per square inch. The field coils have no ventila- 
tion due to their own motion as have armatures, hence 
about iooo circular mils per ampere must be allowed in 
the wire which composes such coils. The cost of copper 
is needlessly increased, if more than the necessary cross- 
section be allowed. 




Fig. 61. 

Field coils are usually wound on brass or iron spools, 
shaped to slip over the cores. Sometimes, especially in the 
case of small machines, the coils are wound on frames, 
which can be collapsed and removed. The coils of series 
machines and the series coils of compound machines are 



FIELD MAGNETS. 



73 



often wound with copper ribbon instead of wire, or are even 
made up of forged copper conductors, having a rectangular 
cross-section. This is because the heavy currents require 
such large cross-section of conductor that if made of wire 
much space would be lost between the wires. The rear 
coil in Fig. 61 is a series coil of shaped conductors. This 
figure shows both the shunt and the series coil, as wound 
by the Westinghouse Company, for a compound multipolar 
railway generator. The binding which is seen on the shunt 
coils in both illustrations should not be mistaken for the 
wires of these coils. Field coils are wound with double 
cotton-covered copper wire. Further insulation between 
coil and core, and between series and shunt coils, is effected 
by the use of fiber, fuller board, and mica. 

47. Magnetic Leakage. — Since air is not an insulator 
of magnetism, but is simply much less permeable than 
iron, it is evident that some of 
the lines of force generated by 
the field coils will not follow 
around the desired path through 
pole pieces and armature, but will 
take a path through the air and 
be of no utility in creating E.M.F. 
in the revolving armature. Fig. 
62 roughly represents some of 
the paths such lines may take. 

If 4> t be the total flux caused by the field coils and <f> a be 
the flux that passes through the armature, then the coeffi- 




Fig. 62. 



cient of magnetic leakage, 



L = 



and is always greater than unity. 



74 



DYNAMO ELECTRIC MACHINERY. 



In practice X. varies from 1.25 to 1.4 in single horse- 
shoe fields, and in the Edison type of inverted horse-shoe 
and in double horse-shoe fields it varies from 1.5 to 1.75. 
In multipolar machines X, varies from 1.1 to 1.5. 

To find the coefficient of magnetic leakage of small or 
moderate sized machines proceed as follows : — 

Arrange the field-coils for separate excitation by a cur- 
rent that can be conveniently commutated. Suppose the 
machine to have a field of the double horse-shoe type, as in 
Fig. 63. Take a few turns of fine insulated wire about 
the middle of one coil, as c, d, and connect the ends to a 




Fig. 63. 

ballistic galvanometer of low sensioility. A low-reading 
Weston voltmeter will answer. Suddenly commutate 
the current in the field coils. The change in direction of the 
flux in the core, from + <f> to — <f>, will induce E.M.F. in the 
test coil, which will give a throw to the voltmeter needle. 
The deflection is directly proportional to the flux in the 
core. Repeat with the other coil, and the sum of the de- 
flections obtained from cd and ef is directly proportional 
to the total flux produced <f> t . Now make a test coil of the 
same number of turns and of the same resistance about 
the armature, in such a position ab that it includes the area 



FIELD MAGNETS. 75 

of the armature that is cut by the greatest number of lines 
of force. Upon commutating tr e current a throw of the 
needle will result, which is propational to the flux in the 
armature cf> a . Hence the coefficient of magnetic leakage, 

. _ <h _ defl. at cd -f- den. at ef 

4> a deflection at armature 

The exciting current must remain constant during the 
investigation. 

The location of the different leakage paths may be found 
by using test coils on different parts of the frame. The 
difference between the throws observed at any two places 
is a measure of the leakage between those two places. 

Clearly the number of lines choosing paths through the 
air will decrease as the permeability of the iron circuit 
increases. An increase in the reluctance of the main 
magnetic circuit will increase the leakage loss. 

Armature cores vary in permeability under varying con- 
ditions of load. As the load increases, this change pro- 
duces an increase in the reluctance of the main magnetic 
circuit. This results in an increase of the loss by leakage. 
The coefficient of magnetic leakage is, therefore, different 
with different loads. 

48. Pole Pieces and Shoes. — In general practice the 
field cores and the frame of a generator are worked at a 
flux density of at least 15,000 lines per sq. cm. 

This is too high a value to use in the air gap. Therefore 
pole shoes are put on the ends of the pole pieces to dis- 
tribute this flux over a wider area where it has to pass 
through the air, and to thus decrease the total reluctance 
of the magnetic circuit. 



76 DYNAMO ELECTRIC MACHINERY. 

49. Effect of Joints in the Magnetic Circuit. — Since no 
two pieces of metal can be put together with a perfect 
joint, there is always an increase of reluctance in a mag- 
netic circuit when a joint is introduced therein. Professor 
Ewing found by experiment that at low magnetizations 
(JC = 7.5) the increase of reluctance of a certain bar of 
iron due to a joint was above 20 per cent, and that for 
high magnetizations (3C = 70) the loss due to one joint was 
less than 5 per cent. The difference is probably due to 
the fact that the pieces under strong magnetizations attract 
themselves so powerfully as to make a more perfect joint. 
Ewing also found that a single cut in a bar acted upon the 
reluctance of the bar as though the length of the bar had 
been increased by amounts given in the following table : — 

ForX = 7.5 15 30 50 70 

Equivalent length of 1 cut 

in cms. of iron ... 4 2.53 1.10 0.43 0.22 



OPERATION OF ARMATURES. 77 



CHAPTER V. 

OPERATION OF ARMATURES 

50. Process of Commutation. — The simple process of 
commutation as described in § 30 is attended with some 
difficulties in practice. Consider one coil of a plain ring 
armature with the commutator bars attached thereto as in 
Fig. 64. In position A, when the brush is on only one of 
the bars in question, the action of the other coils of the 
armature will be to force current in this one coil in the 
direction indicated by the arrow. B is considered to be 






the positive brush. In position D, when the brush has 
passed over to the other bar entirely, the direction of the 
current in this coil is in the other direction. Now this 
change of direction must occur when the coil is in a weak 
field, for it is observed that the coil is short circuited while 
in position C, the circuit being completed through the coil, 
the bars and the brush spanning the mica insulation at o. 
If now at this moment the coil should be in a strong field, 
and should be cutting many lines of force, too large an 



78 



DYNAMO ELECTRIC MACHINERY. 



E.M.F. would be produced, and as the resistance of the 
circuit indicated is very low, an excessively strong current 
might flow. When the brush slips past o the circuit is 
broken, and a more or less serious sparking occurs accord- 
ing to the strength of the current flowing at the instant of 
break. Commutation must then be effected when the coil 
is in such a position as not to cut many lines of force. It 
follows that every commutating machine must have at 
least two places where the effective field has a zero value. 
Fig. 65 gives a rectified curve of the magnetic distribution 



£~\ 



Fig. 65. 



under the pole pieces and around the armature of a well- 
designed bipolar machine, the ordinates of the curve giving 
the flux density in the air gap. 

The neutral plane is a plane passed through the axis of 
the armature and a point in the field immediately surround- 
ing the armature, where the inductively effective com- 
ponent has a zero value. The coil in position C } Fig. 64 
is supposed to be in the neutral plane. 

The commutating plane is a plane passed through the axis 
of the armature and through the points of contact of the 
brushes. The segments are supposed to be connected with 
parts of the armature windings lying on the same radius. 



OPERATION OF ARMATURES. 79 

51. Influence of Self-Induction of the Commutated 
Coil. — When the coil in Fig. 64 is in position A the cur- 
rent flowing in it produces magnetic flux in the ring inde- 
pendent of any inductive action of the field magnets of the 
dynamo, and links the flux with itself. When the coil is 
in position D, there is also a magnetic flux and linkage, but 
its direction has been changed. Therefore, in passing 
through the position C, the current in the coil and the 
accompanying flux linked with the coil have decreased to 
zero, and have afterwards risen in value in the opposite 
direction. 

This change of flux produces an E.M.F. i7i the coil inde- 
pendent of any action of the field magnets (see §15). This 
E.M.F. is called an electromotive force of self-induction and 
tends to continue the flow of a current which has been 
started, and tends to prevent any increase or decrease in 
the strength of the current and to prevent the stopping or 
starting of the current. The value of this self-induced 
pressure with a given flow of current varies as the square 
of the number of turns in the coil, as the cross-section of 
the coil, and as the permeance of the magnetic circuit. 
Because of self-induction it is evident that, if commutation 
take place in the neutral plane, there is a liability that it 
will be accompanied by excessive currents in the short-cir- 
cuited coils and consequently by sparking. This trouble 
is to be avoided by revolving the plane of commutation 
about the shaft of the machine until a sufficiently strong 
field acts upon the short-circuited coil to induce an opposing 
E.M.F. of the same value as the E.M.F. of self-induction. 
Both the self-induced E.M.F. and the E.M.F. due to the 
rotation of the armature vary in magnitude during the 
time that a brush is upon two adjacent segments. Their 



8o 



DYNAMO ELECTRIC MACHINERY. 




Fig. 66. 



manners of variation need not be alike, and hence it may 
be impossible in some cases to effectively oppose one 

against the other. The 
obvious remedy is to be 
sought in more com- 
mutator segments or a 
change of shape of pole 
shoe. 

52. Cross-Magnetiz- 
ing Effect of Armature 
Currents. — Indepen- 
dent of field magnets 
the current flowing in 
the armature conductor 
will magnetize the ar- 
mature core. The poles thus produced will be in the 

plane of commutation. Fig. 66 shows the magnetizing 

effect of the armature turns 

on a ring armature. Fig. 

6y shows a cross-section of 

a drum armature and its 

windings with the resulting 

magnetization. 

Thus, when there is a 

load on a dynamo and the 

armature conductors are 

carrying a heavy current, 

, . Fig. 67. 

there are two coexistent 

magnetic fields. This condition results in a skewing of 
the lines of force, as is shown in Fig. 68. As the lines 
are skewed the neutral plane is shifted. To produce spark- 




OPERATION OF ARMATURES. 



8l 




less commutation the commutating plane must also be 
shifted. This causes a further skewing of the lines. The 
limit of this double interdependent shifting is reached 
when the magnetic lines have 
become so crowded in the 
trailing-pole tips that they are 
almost insensible to a further 
shifting of the plane of com- 
mutation. mg - 68 - 

. This skewing is a source of loss in the operation of a 
generator because it increases the magnetic reluctance in 
two ways, — (a) by saturating the iron at the horns, and 
thus reducing the permeability, and (b) by lengthening the 
paths, both in air and in iron, that the lines must follow. 

Fig. 69 shows a 
curve similar to Fig. 
65 taken when the gen- 
erator was under load 
and the armature was 
traversed by a heavy 
current, the flux being 
distorted because of it. 
It is evident that the 
angular displacement of 
the neutral plane depends in magnitude upon the relative 
number of armature ampere turns as compared with the ef- 
fective field ampere turns. The use of a strong field and a 
large air-gap length requires a large number of field ampere 
turns. Both are much used in practice with great success. 




Fig. 69. 



53. Demagnetizing Effect of Armature Currents. — It 

has been shown that it is necessary to have the commu- 



82 



DYNAMO ELECTRIC MACHINERY. 



tating plane in advance of the neutral plane. The angle 
between them is called the angle of lag or lead. If an 
axial plane be passed through the armature, making with 
the neutral plane an angle equal to the angle of lag or lead, 
but on the opposite side of the neutral plane from the com- 
mutating plane, then the angular space between this plane 
and the commutating plane is called the double angle of 
lag or lead. The armature conductors, which create a 
magnetism that tends to skew the lines of the field magnets 
as shown in the last article, are called the cross turns. 




Fig. 70. 

They lie outside the double angle of lead. Those armature 
conductors which lie within the double angle of lead are 
called the back turns, because, when carrying a current, 
their magnetic tendency is to send lines in a direction 
exactly opposite to the lines of the field magnets. They 
neutralize in a certain measure the action of the field turns. 
This action is clearer shown in Fig. 70, which is a cross- 
section of a bipolar drum armature. At a there is a north 
pole due to the back turns which lie in the double angle, 
and at b there is the corresponding south pole. The effect 



OPERATION OF ARMATURES. 83 

of these poles is to neutralize some of the useful magnetic 
lines flowing from N to S. At c there is a south pole 
due to the remaining or cross armature turns and at d is 
the corresponding north pole. These poles skew the lines 
flowing from N to S. Compensation for back turns is 
easily calculated, since the number of back turns times 
the current in them at any load multiplied by the coeffi- 
cient of magnetic leakage at that load (§ 47) gives the 
number of additional field ampere turns necessary at that 
load for compensation. 

54. Sparking. — As shown in § 51, sparking can be 
avoided by giving the brushes a lead sufficient to bring the 
coils they short circuit into fields sufficiently strong to coun- 
teract the effects of self-induction. Sparking in the opera- 
tion of machines is generally due to the misplacement of the 
brushes, though sometimes it is due to irregularities of the 
commutator surface. A high bar passing from under a 
brush will leave the latter suspended in air a moment, which 
will break the whole current through 
the brush and cause a bad spark or 
arc. 

A machine may also suffer melting 
of the commutator bars without any 
visible sparking. Suppose a coil of 
low resistance to be short circuited 
by a copper brush as in Fig. 71. 
When the brush is chiefly on one 
bar, and over-laps the other very 
slightly, then a very considerable part of the resistance in 
the circuit is the transition resistance at the small contact. 
Under an E.M.F. of self-induction a current of sufficient 




84 DYNAMO ELECTRIC MACHINERY. 

magnitude may flow to produce enough heat in the trans- 
ition resistance to melt the surface of the commutator bar. 
The E.M.F. may then disappear before the brush leaves 
the bar, and there will be no spark visible. 

Sparking may be due to excessive electromotive force 
between the commutator segments undergoing commuta- 
tion due to the self-induction of the coil and to mutual 
induction between it and other coils undergoing commuta- 
tion at the same time. To be able to determine the value 
of this induced E.M.F. one must know both the self and 
mutual inductances, and the time rate of suppression of the 
current in the coil. Parshall and Hobart state that in 
practice one may assume that a coil of a single turn when 
traversed by one ampere produces and links with itself 
20 c.g.s. lines per inch net length of armature lamination. 
From this datum one can calculate the values of the self- 
inductance and mutual inductance. 

A coil which is undergoing commutation must have its 
current changed from a maximum value in one direction to 
zero and from zero to a maximum value in the other direc- 
tion during the time that the two segments at its ends are 
connected through the brush. This time is evidently 
dependent upon the peripheral speed of the commutator 
and upon the width of the brush. It is equal to the time 
that it takes a point of the insulation between the seg- 
ments to pass over the breadth of the brush ; that is, the 
time in seconds is equal to the breadth of the brush in 
inches divided by the peripheral velocity of the commutator 
in inches per second. The reciprocal of this time gives the 
number of commutations per second, or what is termed 
the frequency of commutation. The frequencies found in 
practice lie between 200 and 500 per second. While all 



OPERATION OF ARMATURES. 85 

the current which traverses the coil is suppressed in one- 
half the time taken for commutation, the manner of its 
variation is unknown. Parshall and Hobart assume that 
the current strength falls sinusoidally. An assumption of 
a uniform decrease with the time yields results quite in 
accord with practice. The value of the induced voltage 
then will be equal to the product of the value of the com- 
mutated current and the sum of the mutual and self-induc- 
tance divided by one-half the time occupied in completing 
commutation. This value should not exceed 6 volts. 

55. Prevention of Sparking The limit of the capacity 

of a machine may be excessive sparking instead of exces- 
sive heating, and therefore the suppression of sparking by 
proper design of the machine is of utmost importance. 

Sparking may be prevented : — 

a. By shifting the brushes till the short-circuited coil 
is just under the fringe of the pole piece. This counter- 
acts the effects of self-induction as explained in § 51. The 
reversal of the direction of flux in any but the short-cir- 
cuited coils is to be avoided, since a loss of useful E.M.F. 
would then occur. 

b. By having a stiff field, that is, a field so strong as to 
suffer very little skewing because of the armature cross 
turns. There is then no lag. In practice, air-gap magnetic 
densities vary from 2500 to 7500 lines per square centimeter. 
The higher densities are to be found in the larger machines. 
There is a general tendency to increase the density. 

c. By nearly saturating the teeth of the armature core. 
When the core teeth are nearly saturated, an increase of 
load increases the reluctance very markedly, and the demag- 
netizing effect of the back turns is restrained on increase 



86 DYNAMO ELECTRIC MACHINERY. 

of load, because of the greater reluctance of the circuit. 
This minimizes the shift of the commutating plane from no 
load to full load, and is a device invariably employed on 
railway generators and other machines that have to stand 
severe changes of load without change of position of 
brushes. 

d. By using brushes of carbon, brass gauze, etc. In 
machines of over ioo volts, carbon brushes are commonly 
used. Besides their good wearing qualities, their resistance 
prevents the flow of a large current in the short-circuited 
coil in commutation, and thus a misplacement of the 
brushes will not result in so violent a spark. In very low- 
potential machines, as has already been said, carbon brushes 
are impracticable, because .their resistance causes a too 
great fall of potential. So in these machines copper strip 
brushes are employed when possible. When too much 
sparking occurs with plain copper brushes, a brush of some- 
what greater resistance is employed, such as copper gauze, 
brass, brass gauze, etc., according to the requirements of 
the case. 

e. By slotting the pole pieces longitudinally. This in- 
creases the reluctance offered to the lines due to armature 
reactions, and so tends to prevent sparking. 

f. By properly shaping the pole pieces. The distribu- 
tion of flux should be such that a coil enters a weak field 
first, and so gradually comes to the strongest part. If the 
lines of force are allowed to crowd into the trailing-pole 
tips, this gradual transition is impossible. If the horns are 
farther from the armature surface than the body of the 
pole face, then the air gap and consequently the reluctance 
at the horns is increased, and the lines are compelled to 
distribute themselves more symmetrically. A place suit- 



OPERATION OF ARMATURES. 87 

able for commutation is then more readily found. One 
may also resort to the shaping of the pole pieces by champ- 
fering the corners, or by making the pole faces with a cir- 
cle of greater radius than the armature. 

The Sprague Electric Company, in its split-pole type of 
the Lundell generator, avoids the distortion of the field 
under full load, due to cross magnetizing turns, by making 
use of a specially designed pole piece. Fig. 72 repre- 
sents a cross-section of this generator, and shows the con- 
struction of the pole piece. The magnetic flux which 
enters the pole piece, divides between the two paths a and 
b. Owing, however, to the greater span covered by the 
shoe belonging to the part marked b, the magnetic reluc- 
tance of that part is much smaller than that of the part 
marked a. As a result, the flux does not divide itself 
equally between the two paths. The part of the pole piece 
marked b, under increasing excitation becomes saturated 
before the part marked a. At normal excitation, the flux 
density at b is above 16,000 lines per square centimeter, 
while the flux density in a is but about 10,000 lines per 
square centimeter. In other words, b is pretty well satu- 
rated, while a has not been brought to a magnetization as 
high as the knee of the magnetization curve. This satura- 
tion of half of the pole piece is effective in preventing a 
skewing of the field by the cross turns. This is shown in 
Figs. 73 and 74, where Fig. 73 represents the development 
of a 50 kilo- watt Lundell generator, and Fig. 74 shows the 
distribution of flux along the line xy of Fig. 73. The 
dotted line represents the distribution at no load, and the 
heavy line the distribution at full load. This small dis- 
torting effect of the cross turns permits the employment of 
a small air gap without serious sparking. 



88 



DYNAMO ELECTRIC MACHINERY. 




r 


■ 


■/--* 


S Q : 




G L 




3 




□ 




1 pl 




• . 


A 




— 


H 1 


P 


q 




.u 




U 


..,.'..,._,.-. 







Fig. 72. 



1\ttt~V 



" ( — ;■ 



-flflfli 



Iflfflfl 




Fig. 73. 



OPERATION OF ARMATURES. 89 

Ryan compensates for the magnetizing effects of the 
armature winding by surrounding the armature with a sta- 
tionary winding, which passes through perforations in the 
pole faces. These stationary windings carry the whole 
current of the machine. This method prevents all spark- 
ing due to the distortion of the field, but it does not pre- 
vent the sparking which is due to self-induction and mutual 
induction of the armature coils. The latter sparking is 
prevented to a certain extent by inserting a lug between 
the pole horns, which is magnetized by a few series turns. 




Fig- 74- 

56. Energy Losses in Operation. — Besides the energy 
expended in exciting the field coils, there are losses of 
energy in the armature and connections, as follows : — 

a. The bearing friction and the windage. This loss is 
generally considered independent of load, but it is ques- 
tionable whether the friction does not increase somewhat 
under loads. This loss is from 1 5 per cent to 40 per cent 
of the total loss. 

b. The hysteresis loss in the iron of the core due to the 
continued reversal of the direction of magnetism therein. 
According to Steinmetz's Law, the hysteresis loss in watts, 

h = io 7 F(B lc v /i, 



90 DYNAMO ELECTRIC MACHINERY. 

where F'is the volume of iron in cubic centimeters, & the 
flux density, n the number of magnetic reversals per sec- 
ond, and rj a constant depending in value upon the char- 
acter of the iron. A table of values is given on page 29. 
The value of (B varies at different loads and at different 
places, as was shown by Goldsborough, so this loss cannot 
be said to be proportional to the speed or any power of 
the voltage. The hysteresis loss is from 1 5 per cent to 
40 per cent of the total losses. 

c. Eddy currents in the iron and the copper conductors. 
These might be expected to vary as the square of the 
speed, but they do not for the same reason as in b. Be- 
cause of the laminated structure of the core, and the 
slight angular breadth of the conductors, this eddy loss is 
of small magnitude, from 2 per cent to 10 per cent of the 
losses. It may amount to 50 per cent of the losses in 
the case of smooth-core armatures. Eddy currents in the 
pole faces, which may be due to any variation in the re- 
luctance encountered by the lines passing through the 
poles, are reduced by an increase of air-gap length. They 
are greatest with armature cores having slots with large 
openings at the top, and least with armatures whose in- 
ductors are threaded through inclosed channels in the core. 

d. The armature resistance loss. This equals I 2 R, where 
/ is the total current of the machine, and R the resistance 
of the armature measured between points rubbed by the 
brushes which are drawing the current /. This is ex- 
clusive of the transition resistance at the brushes. In 
500 k. w. machines the PR loss is about 2 per cent of the 
total output. In 5 k. w. machines about 4 per cent, and 
in smaller machines much greater. 

e. The friction of the brushes against the commutator. 



OPERATION OF ARMATURES. Cjl 

This loss varies about as the speed, and its importance is 
generally underestimated. Carbon brushes press upon the 
commutator with a force of from 1 to 12 pounds per 
square inch of contact. Railway motors and similar ma- 
chines have the larger value, while central-station gene- 
rators have the smaller. The coefficient of friction between 
carbon and copper varies from 0.28 to 0.32. 

f. The resistance of the brushes and the transition re- 
sistance of the brush contacts. The first loss varies as 
the square of the current, and is of considerable magni- 
tude in low-potential machines. The transition resistance 
seems to vary inversely as the current, thereby always 
causing a constant drop of voltage amounting to from 1 
to 1.5 volts per transition. 

The heat produced by losses b, c, and d, being in the 
armature itself, must be dissipated by the conduction, con- 
vection, and radiation. Experience shows that from 2 to 
2\ watts can be radiated from every square inch of arma- 
ture surface without causing a dangerous rise of tempera- 
ture in the armature core. It is found that about 500 
circular mils per ampere in the armature conductors brings 
the loss d to such a point that, added to the losses c and 
b, they together give about 2 watts per square inch of 
armature surface ; hence this value of 500 circular mils per 
ampere is the mean of what is usually adhered to in winding 
armatures of commercial machines. 



92 DYNAMO ELECTRIC MACHINERY. 



CHAPTER VI. 

EFFICIENCY OF OPERATION. 

57. Efficiency The following definition and discussion 

of efficiency is taken from the report of the committee on 
standardization of the American Institute of Electrical En- 
gineers : — 

The "efficiency" of an apparatus is the ratio of its net 
power output to its gross power input. 

Electric power should be measured at the terminals of- 
the apparatus. 

Mechanical power in machines should be measured at' 
the pulley, gearing, coupling, etc., thus excluding the loss 
of power in said pulley, gearing, or coupling, but including 
the bearing friction and windage. The magnitude of bear- 
ing friction and windage may be considered as independent 
of the load. The loss of power in the belt, and the in- 
crease of bearing friction due to belt tension, should be 
excluded. Where, however, a machine is mounted upon 
the shaft of a prime mover, in such a manner that it cannot 
be separated therefrom, the frictional losses in bearings 
and in windage which ought, by definition, to be included in 
determining the efficiency, should be excluded, owing to the 
practical impossibility of determining them satisfactorily. 
The brush friction, however, should be included. 

Where a machine has auxiliary apparatus, such as an ex- 
citer, the power lost in the auxiliary apparatus should not 



EFFICIENCY OF OPERATION. 93 

be charged to the machine, but to the plant consisting of 
machine and auxiliary apparatus taken together. The 
plant efficiency in such cases should be distinguished from 
the machine efficiency. 

The efficiency may be determined by measuring all the 
losses individually, and adding their sum to the output to 
derive the input, or subtracting their sum from the input 
to derive the output. All losses should be measured at, or 
reduced to, the temperature assumed in continuous opera- 
tion, or in operation under conditions specified. 

58. Coefficient of Conversion. — This has sometimes 
been called the efficiency of conversion, but because of the 
definition of the last paragraph it is better not to use the 
word efficiency. The coefficient of conversion fi is the ratio 
of the total electrical energy developed in the armature 
winding to the total mechanical energy expended, 

B - ItE ' 

where P is the power expended in watts, I t the armature 
current in amperes, and E t the E.M.F. in volts, developed in 
the armature. (3 is always less than unity, because of the 
friction and windage of the armature, because of the eddy 
currents in the core and conductors, and because of the 
hysteresis of the core. 

59. Economic Coefficient. — (rj) This coefficient is equal to 
the ratio of the useful electrical energy to the total electri- 
cal energy developed in the armature circuit. It is always 
less than unity because of the necessary loss of energy in 
the exciting coils and in the armature coils. In the case 
of a series dynamo, if we let 



94 DYNAMO ELECTRIC MACHINERY. 

E t = E.M.F. generated in volts, and 
E = Terminal pressure in volts, then the economic coef- 
ficient for a current of / amperes is 

IE _ E 
r, -/E t -'E t ' 
For shunt dynamos, 

IE 

Where /is the current in the outside circuit, I f the current 
in the field coils, E the pressure at the terminals of the 
machine, and E t the total pressure generated. 

The efficiency of a machine e is evidently the product of 
ft and 77. 

For a series machine, I — I t and 

I t E t E IE 

e = ^=— X E t =^' 

For a shunt machine, I -\- I f = I t , and 

I t E t IE IE 

< = (!,= -- Xj —^ = T . 

Hence the product of (3 and -q for either machine is the 
same, and corresponds to the definition of efficiency. 

60. Separately Excited Dynamos. — At a constant speed 
and constant exciting current a nearly constant total pres- 
sure (E t ) is generated; and it is almost equal to the pressure 
at the terminals at no load — that is, on open circuit. This 
follows from the equation for the average pressure, 

VS+p 



60. 1 o 8 



(§33) 



where — is the number of revolutions per second, 5 the 
60 

number of inductors, <f> the flux per pair of poles, and / the 



EFFICIENCY OF OPERATION. 



95 



number of pairs of poles. If the speed be varied, the pres- 
sure will vary proportionally if no load is on the machine. 
If, however, a current be taken off, then the demagnetizing, 
effects of the armature currents become evident in a 
change of the value of <f>, and there will be a falling off of 
pressure. The amount of this deviation is dependent upon 
the composition and saturation of the magnetic circuit. 



































100 

80 






























40 






























iiU 


* 
















} 1 


2 


3 


i 
MVIPERES 


5 


6 


a 



Fig. 75- 



This effect is clearly seen in the curve in Fig. 75, where 
the armature currents are measured in the X direction, and 
the pressure in the Y direction, the conditions of speed and 
exciting current remaining constant. 

Let E t = the total volts produced, 

E = the volts at the terminals of the machine, 
R a = the resistance of the armature, 
R = the resistance of the external circuit, and 
/ = the current under these conditions. 



g6 DYNAMO ELECTRIC MACHINERY. 

Then for a separately excited machine, 

E t =I{R + R a ), 
E = IR, and 

EI I 2 R I 



V = 



E t I I* (R + R a ) R + R a 
and 

* B, 



R+R< 



In determining the efficiency of a separately excited 
machine the energy lost in the exciting coils must be 
charged against the coefficient of conversion. 

The operation of any dynamo can best be studied by in- 
spection of a curve which shows the relation existing be- 
tween the current generated or supplied by the machine, 
and the voltage under which it operated. Such curves 
are called Characteristic Curves^ and they are generally 
plotted with currents for abscissae and volts for ordinates. 
The characteristic curve for a separately excited dynamo 
is that shown in Fig. 75. 

61. Magnetos. — A separately excited dynamo whose 
field is maintained by a permanent magnet, instead of an 
electric magnet, is called a magneto. These machines 
from their similarity, both theoretically and practically, 
should be mentioned together. Magnetos are, however, 
generally alternating current machines with slip rings in- 
stead of commutators. They are used in very great num- 
bers in telephone subscribers' sets, and in many electrical 
businesses for testing out the continuity of concealed con- 
ductors, and in some cases for determining defective 
insulation. To the armature is affixed a pinion, meshing 
with a gear turned by hand. The alternating current pro- 



EFFICIENCY OF OPERATION. 



97 



duced is passed through the circuit whose continuity it is 
desired to determine, and then passes through a polarized 
bell which is caused to ring 
These machines are manufac- 
tured so as to ring through 
an external resistance of as 
high as 50,000 ohms without 
undue effort at the handle. 
The cut Fig. 76 shows a com- 
mercial belt -driven magneto. 



62. Series Dynamos. — 
Letting E, E t , R, R a , and / 
have the same significance as 
before, represent by R f the 




Fig. 76. 



resistance of the field winding, and by R b the resistance 
of the brushes and transition contacts. Then 

E = IR, 
E t = I(R + R f + R a + R b ), 

whence it follows 

EI I 2 R R 



' EJ I*{R + R a +R h + R f ) R + R a +R b +R f 

The value of -q increases as R uy R b , and R f approach zero. 
R h is liable to be of greater importance than is imagined. 
In low-tension machines all the resistances are small, and 
care must be taken that R b does not unduly increase the 
denominator of the expression for -q ; in other words, cop- 
per brushes should be used on low-voltage machines. 

The value of 77 varies as R, but the load varies in- 
versely as R ; hence rj is a maximum when the load is a 



9 8 



DYNAMO ELECTRIC MACHINERY. 



minimum, and rj = i when R = oo, or there is no load. 
Fig. 77 is a curve showing relation between rj and load. 




63. Characteristic Curve of a Series Machine. — Fig. 
78 shows the curves of a series dynamo. The curve of 
total volts E t is very similar to the magnetization of a 

magnetic circuit made 
up of iron chiefly. It 
falls below such a 
curve (a) because 
saturation causes in- 
creased magnetic leak- 
age, and hence the 
value of <f> in the 

equation^ = ^-^ 

is not proportional to 
the total flux, and 
(b) because of the demagnetizing and cross magnetizing 
effects of the armature currents. The curve E, starts, 
above zero because of the residual magnetism in the cores 
of the field magnets. If operated under constant load, a 




EFFICIENCY OF OPERATION. 99 

series dynamo will give E, directly proportional to the, 
speed. 

The straight line represents the loss or drop of potential 
due to the resistances of the machine, R a , R b , and R f . 
Since drop of potential is proportional to the resistance, 
this is a straight line, and must pass through the origin. 
This loss line can be established by a point found by as- 
suming the lost volts Ei and solving for the current I from 
the equation I (R a + R b + R/) = E z . For example, if the 
resistances R ft -f- R*+ R/ be assumed as 0.2 ohm, then 10 
volts would be lost in them only when 50 amperes were 
flowing through them. A line drawn through the origin, 
and a point on the characteristic curve diagram whose 
coordinates were 10 volts and 50 amperes, would at every 
point give the volts lost in sending the corresponding num- 
ber of amperes. 

The curve E showing the E.M.E. at the terminals of 
the machine as a function of the current output is found 
by subtracting the ordinates of the loss line from those of 
E t and using the differences as the ordinates of E. In 
practice E t cannot be directly found ; but the terminal volts 
and the current can be measured, thus giving the curve E y 
and from a knowledge of the loss line the curve E t can be 
derived. 

The operation of some special forms of series machines 
will be discussed in the chapter on arc-lighting machines, 

64. Power Lines. — Where volts and amperes are used 
as ordinates and abscissae, lines can be drawn connecting 
points of constant product of the two, representing watts 
or power. Fig. 79 shows such lines drawn for one, two, 
and three kilowatts. If E be the external characteristic of 
LofC. 



IOO 



DYNAMO ELECTRIC MACHINERY. 



a dynamo, then the curves make it apparent that the ma- 
chine cannot generate 3 k.w., but that for most values 

under 3 .k. w. 
there will be two 
loads under which, 
the generator can 
run and yield the 
same voltage. 



65. Shunt Dy- 
namos. — In 

shunt-wound ma- 
chines the cur- 
rent in the arma- 
ture is the sum of 
the current in the 
field coils and of 
that in the ex- 
ternal circuit, or 

















eV— 




<? 

0\£ 


















^* 















AMPERES 

Fig. 79- 



T a = I f + I. For sake of simplicity we will assume I a — I. 
Practically this introduces but a small error under ordinary 
conditions of load. 

E 2 
IE PR R 



V = 



IE t + IjE I 2 (R + R a ) + I/R f 



E' 2 
~R 



E 2 R 
R 2 



E 2 



I 

R 



I R a I 

R R 2 R t 



1 a 1 

I 4--7T + 



R 



R R ( 



To determine what value of R will enable a given ma- 
chine to operate with a maximum economic coefficient — 



EFFICIENCY OF OPERATION. 



IOI 



place the differential coefficient of y, in respect to R con- 
sidered as a variable, equal to o and solve for R 



dt] 



R, 



| = o .-. R = VRJ? f . 



dR R f R 

The external resistance must be a mean proportional be- 
tween R a and R f , and the maximum economic coefficient is 

i 



V = 



i + ^y- 



Ra 
R f 



66. Characteristic Curve of a Shunt Dynamo. — In Fig, 
80 the curve E is plotted from experimental results obtained 
while the machine is running at various loads. To get satis- 




factory results, one should begin with an infinite resistance 
in the external circuit, which is then reduced step by step, 
In some small machines it can be reduced to zero without an 
extreme elevation of temperature due to excessive currents. 
As a rule, only the upper and lower. values of E, correspond- 
ing to currents between o and a definite maximum value, can 



102 DYNAMO ELECTRIC MACHINERY. 

be obtained. The loss line L is obtained by calculation as 
before in the case of the series machine. The curve 
showing the relation between external current and total 
volts, E t , is obtained by adding the ordinates of L to those 
of E. The drop in E is at first due chiefly to the drop re- 
sulting from armature resistance. As the current increases, 
the effects of armature reaction and saturation of the 
magnetic circuit become evident. At the same time E is 
affected by a decrease of the shunt-field current due to 
the fall of potential at the terminals of the field circuit. 
This soon becomes the predominating cause of drop, and to 
such an extent that the curve turns back toward the origin. 
When zero resistance is in the external circuit, of course no 
current flows through the field, and the few volts then 
produced are due to residual magnetism. It must be 
remembered that while E is a double-valued function of / 
it is a single-valued function of R. 

The voltage of a shunt machine generally increases more 
rapidly than the speed. An increase of speed not only in- 
creases primarily the number of volts generated, but also 
increases the armature flux <f> because of increased excita- 
tion. The condition of the magnetic circuit as regards 
saturation determines whether this secondary influence 
shall be great or small. 



CONSTANT POTENTIAL DYNAMOS. 103 



CHAPTER VII, 

CONSTANT POTENTIAL DYNAMOS. 

67. Constant Potential Supply The method of sup- 
plying, at any point of usage, current at a constant poten- 
tial irrespective of the load which is there or elsewhere, is 
used in the distribution of electrical energy for purposes 
of incandescent electric lighting, for consumption in con- 
stant pressure motors, and for trolley-car propulsion. The 
great sensitiveness of the candle power of incandescent 
lamps to a change in voltage, the candle power varying 
as the fourth power or more of the voltage, requires 
that the pressure in lines used for lighting must not vary 
by more than 3 per cent of its rated value. In street -car 
work, where the load suffers tremendous variations, con- 
stant potential supply is equally as imperative for satisfac- 
tory operation. 

68. Methods of Obtaining Constant Potential For 

accomplishing this result many devices have been tried, 
the more important of which are : — 

a Automatic variation of the resistance in the field cir- 
cuit of shunt machines. 

b Automatic change of the position of the brushes and 
commutating plane. 

c Automatic variation of armature speed. 



104 



DYNAMO ELECTRIC MACHINERY. 



d Hand regulation of a resistance in series with a shunt 
field coil. 

e Self-regulation. 

Of these the first three methods are no longer employed, 
and either hand regulation or self-regulation or both to- 
gether are relied upon to maintain the constant voltage 
under varying loads. 

69. Hand Regulation Inspection of the characteristic 

curves of either the shunt or the separately excited dynamo 
shows a drop in the voltage as the load increases. This is 
-due to the internal resistance of the armature and the 
demagnetizing effect of armature reaction. In the formula 

S4>Vp 






for the E.M.F. of a machine, E 



, the only quan- 



HAND 
REGULATOR. 



^^ir™ 



tity that is practical to vary is <f>. This can easily be 

accomplished by regulating 
the amount of resistance in 
a rheostat, which is in series 
with the field coils and which 
therefore governs the amount 
of current in them, as in 
Fig. 81. 

In distributing current for 
use among a number of con- 
sumers the current is carried 
to feeding-points which are 
near the locality they supply, but may be distant from the 
station. It is desirable to keep the pressure at these 
points at a constant value, irrespective of the varying loss 
of potential that is going on because of the resistance of 
the conductors leading to them. To achieve this end the 



fO 



4- 

Fig. 81. 



CONSTANT POTENTIAL DYNAMOS. 



105 



Edison system employs feeders to carry the current to the 
feeding-points. Each feeder is accompanied by a pilot wire 
imbedded in the insulation. At the feeding-point the pilot 
wires are attached to the feeder terminals, and at the sta- 
tion end are attached to a voltmeter, so that one can, in. 
the station, regulate the pressure not at the machine ter-, 
minals but at the distant distributing point. 

70. Field Rheostats For varying the current in the 

shunt fields of dynamos, it is usual to employ field rheostats > 




Fig. 82. 



which are mounted on the switch-board along with indicat- 
ing instruments. A form of such rheostatic regulators is 
the so-called Packed Card Rheostat, manufactured by the 
General Electric Company. This derives its name from 



o6 



DYNAMO ELECTRIC MACHINERY. 



the method of constructing it. A tube of asbestos, in- 
closing a steel mandrel, is wound with a chosen amount of 
German-silver wire or ribbon. The tube is then removed 
from the mandrel, and pressed into the form of cards as 
shown in Fig. 82. These cards are then assembled, with 
interposed asbestos, in sufficient numbers to make up the 
required resistance of the rheostat. Iron plates, somewhat 



1. 4m9^b9^@£ 


1 






^^^/* 



Fig. 83. 

wider than the cards, are introduced at intervals, and thus 
increase the radiating surface. The whole is held together 
by iron end plates and bolts, as shown in Fig. 83. Con- 
tact bolts are connected with various points of the conduc- 
ting part of the rheostat, and these bolts are connected 
through a wiping-finger with the field circuit. Fig. 84 shows 
a rheostat of this type built for regulating a railway gene- 
rator and arranged to be placed on the back of a switch- 



CONSTANT POTENTIAL DYNAMOS. 



107 



board with the regulating handle projecting in front. 
For the largest generators resistances made of iron grids 
supported in iron frames are employed. Both of these 
constructions are fire-proof and easily repaired in case of 
accident. 




Fig. 84. 

When large generators, such as are used in railroad work, 
have their field circuits opened, the E.M.F. self-induced 
by the disappearance of the flux in the fields is liable to 
reach such a magnitude as to pierce the insulation of the 
field coils and destroy their usefulness. To obviate this, 
before the field circuit is broken, the field coils are con- 
nected (Fig. 85) through a high discharge resistance, and 
the current in them is allowed to die out slowly. It is 
thus unattended with any destructive potential differences. 



io8 



DYNAMO ELECTRIC MACHINERY. 



The Edison Electric Illuminating Company of New York 
City, in the case of its Duane-street generators, allows the 
field circuits to discharge themselves through an arc light. 
Another form of field rheostat is the Carpenter Enamel 
Rheostat, made by the Ward Leonard Electric Company. 
In this rheostat the heat generated is not radiated directly 



Binding) Posts 




Field Switch 




Field 



Armature 



Fig. 85. 



from the surface of the wire, but is conducted to a sup- 
porting plate, which then becomes the radiating surface. 
The resistance wires are surrounded with an enamel, which 
attaches them to the supporting plates, insulates them 
therefrom, and protects them from corrosion. Owing to 
the increased radiating surface thus obtained, a shorter and 
smaller wire can be used for a given volt-ampere capacity 



CONSTANT POTENTIAL DYNAMOS. 



109 



than if the wire were merely exposed to the air. No con- 
sideration of the mechanical strength of the wire enters 




Fig. 86. 



into the design of this resistance, since it is supported and 
protected by the enamel. To further increase the radiat- 




Fig. 87. 

ing surface, the back of the plate is provided with raised 
annular ribs. The makers claim that this rheostat can radi- 



no 



DYNAMO ELECTRIC MACHINERY. 



ate 5 watts for each square inch of one surface. Thus a 
plate 10 by io inches will dissipate 500 watts. The 

method of using iron radiat- 
ing plates for purposes of 
dissipating large amounts 
of heat is to be found in 
the rheostats of many man- 
ufacturers. Wirt (Fig. 88) 
incloses resistance wire or 
ribbon in radiating plates, 
insulating them from each 
other by means of mica. 
Other firms employ sand as 
an insulating material. 



71. Self -Regulation. — 

By far the most elegant 
method of constant poten- 
tial regulation is that in 
which the main current of • 
the machine is utilized in 
maintaining constant the magnetic flux <f> through the 
armature. This is accomplished by passing all or the 
greater part of the current produced in the armature a 
few times around the field magnets, so that an increased 
load on the armature increases the magnetizing ampere 
turns of the field coils. These series turns, when rightly 
proportioned, can be made to compensate for a part, for 
all, or for even more than all of the drop. This device 
can be used in connection with any other form of 
excitation, as permanent magnets, separate excitation, 
or shunt excitation. In the last case, the dynamo is 




Fig. 88. 



CONSTANT POTENTIAL DYNAMOS. Ill 

said to be compound wound, as described in § 45. If 
the machine is designed to maintain a constant pressure 
at some distant feeding-point, instead of at the machine 
terminals, the machine is said to be over-compounded, since 
the potential at the terminals will rise on increase of load. 
From 3 to 5 per cent over-compounding is frequent in 
machines used to supply lighting circuits, and 10 per cent 
over-compounding is usual in railway generators. 

72. Economic Coefficient of a Compound Machine. — 

To discover the value of rj in this case, let R be the resis- 
tance of the external circuit, R s the resistance of the series 
turns, R sh the resistance of the shunt-field, and R a the 
resistance of the armature. Then assuming that the cur- 
rent in the armature is the same as in the external circuit, 
an assumption which is warranted in the case of commer- 
cial machines, 

rR 



V 



I 2 R 4- I 2 R U + R 2 Rs + S\R S 









1 














R 








T 




T 




R 




R 





+ 




• + 




-f- 




R 




R sf 


1 


R 1 




R l 



R_ R. R, 
^ R sh ^ R ^ R 



Considering R as a variable dependent on rj, and solving 
for a maximum of -7 

^L — 4- — * ; -4- *1 - n 

dR R sh "*" R 2 ^ R 2 ' 

and, 

*= V(ip a + r s ) r„; 

Hence it is seen that the maximum economic coemcient is 
obtained, when the external resistance is the g-eometric 



112 



DYNAMO ELECTRIC MACHINERY. 



mean between the shunt-field resistance and the sum of 
the resistances of the series field and of the armature. 
Under these conditions, 



V = 






R. 



+ 



R. 



i 



I +2 



r-^- 



73. Efficiency of Compound Machines The efficiency 

of- a generator increases with the size, being quite low on 
small machines, and sometimes very high on the larger 
dynamos. Since the distribution of the magnetic and elec- 
trical losses of a generator lies within the discretion of the 
designer, it is possible to so design a machine as to have 

its point of maximum effi- 
ciency at full load or at a 
smaller load, for instance, 
at one-fourth load. The 
two following cuts show 
the relations between effi- 
ciencies and loads on two 
different machines. 

74. The Compounding 
Rectifier. — The gradual 
saturation of the fields of 
a generator as full load 
approaches causes the 
E.M.F. of even a com- 
pound-wound machine to sag at full load, or if the machine 
is so heavily compounded that it maintains its potential at 



LOU 




















1 


1 








90 
























1 






' ' 










s 
























80 








7 
































r 


























70 






/ 
































/ 




























00 
50 
40 


































> 
<J 
z. 

.0 

UJ 


- 




EFFICIENCY CURVE 

200 K.W. SIZE 224 

DIRECT DYNAMO 

SPEED 150 R P. M. 

CROCKER-WHEELER ELECTRIC CO. 
.Ampere, N.J. 


- 


- 


- 






30 




1 


1 


1 


















- 








1 


1 




















20 








1 


t 




















































10 




















) 
































J 





























































1 
























1 


OUT-PUT 


Kl 


.0- 


.■/.-■■ 


It? 




4D4 





- 



£0 80 ,120 160 .200 m » 
Fig. 89. 



CONSTANT POTENTIAL DYNAMOS. 



113 



full load, its voltage will rise abnormally at some load less 
than full load. To counteract this effect, the Crocker 
Wheeler Company employs a device which is termed a 
compounding rectifier. It consists of a suitable resistance 
shunted across the ter- 
minals of the series field r 
coils. The full armature 
current therefore divides 
between this rectifying 
coil and the series coils. 
As the load increases, 
more current passes 
through each, but the 
coils are so designed 
that this increase heats 
the rectifier and causes 
its resistance to increase, 
while the resistance of 
the series coils remains 
practically unaltered. 

Thus, as the load increases, a larger proportion of the whole 
current passes through the series coils, and this compen- 
sates for the sag in voltage that would otherwise have 
existed. 

70. Theory of Self-Regulation. — To determine the 
number of turns of wire necessary to be used in the series 
regulating coils which are wound on the field magnets of a 
compound machine, 

Let n = number of shunt turns. 
n' = number of series turns. 
B = number of back turns. 




100 120 140 160 
Fig. 90. 



114 DYNAMO ELECTRIC MACHINERY. 

X ' = number of cross turns. 
<R a + s ~ the resistance of the armature plus that of the 
series coil. 
I sh = current in the shunt coils. 
/ = current in the armature and also in the series coils, 
since they are practically the same. 
E t = total pressure developed. 
E = pressure at terminals. 
A == the coefficient of magnetic leakage. 
(R == the reluctance of magnetic circuit when armature 
is idle. Then 



^ \/x/ 2 + nl\ h 

(K- = reluctance with current y in armature. 

Let </>, </>'> <\>" > — A ux m the armature under different con- 
ditions of working. 

When no current flows in the armature, 

±TrnI 8h nl sh 

CD = = I.2C • 

When the current I flows in the armature, 



I - 2 5 („r ,~'r z>^_ l - 2 S 



<*>' = =±f— -(nI sh +n f I-BI) = -^a[nI sh +?i f I-BI^ 



where -= - -, hence a represents the ratio of the 

* nl sh 

reluctance at no load to the reluctance with the load / 

The latter value is the greater because of the skewing 

effect of the cross turns, a, therefore, is less than i . 

The flux in the armature which is due to the shunt coils 

only, when a current / flows in the armature circuit, is 



CONSTANT POTENTIAL DYNAMOS. 1 15 

Thus under load the amature flux due to the shunt coils is 
decreased in the ratio, 

<$>" : <j> : : a : 1. 

The series turns must make up this loss, and also compen- 
sate for the loss due to the back turns and for the electri- 
cal losses due to the resistances of the armature and the 
series coils. 

XT 77 VS & A 77' VS $'P 

Now, E t = — ^ , and E t = — ^- , 
io 8 6o io 8 6o 

and E = E\ — fB a + s 

For convenience let 
1.25 VSp 



(RX X io 8 X 60 



then E = ka?il sh -f- [ka (11' — B) - E a + ^ 1. 



The first term of the right-hand member can be written 
knl sh — k (1 — a) ul sh , in which the expression knl sh repre- 
sents the total voltage developed by the machine at no 
load, which is therefore the terminal voltage at that load, 
or in other words is the voltage for which the machine is 
to be compounded. The equation for the terminal voltage 
at the load / therefore becomes 

E = knl sh -k(i-a) nl sh + [ka («' - B) - £ a+s ] I. 
Evidently, if E is to equal knl sh at any and every load, 

- k (1 - a) ?il sh + [ka {11' - B) - E a+S ] f=o, 
whence 

n = — + B H - 

a 1 ka 



Il6 DYNAMO ELECTRIC MACHINERY. 



I 71 1 

Remembering that -7 = -=- and also that the percentage 
of electrical energy loss in the field/ = -y 100, 

ft = pn + B + 



ioofl aE 

In this value for ri the first term gives the number of 
series turns required to overcome the skewing due to the 
cross turns ; the second term gives the series turns neces- 
sary to compensate for the armature back turns ; and the 
third term shows the number of series turns to balance the 
loss due to the resistances of the armature and the series 
coils. 

The difficulty of applying this formula lies in finding a 
suitable value for a. This differs in different machines, 
having according to Jackson a value of from .75 to .85 at 
full load. It is of course dependent on the load, and has a 
value of unity for no load. 

76. Views of the American Institute of Electrical 

Engineers The following statements concerning the 

regulation of direct current apparatus are taken from the 
report of the Standardization committee of the Insti- 
tute : — 

The regulation of an apparatus intended for the gene- 
ration of constant potential, constant current, constant 
speed, etc., is to be measured by the maximum variation 
of potential, current, speed, etc., occurring within the 
range from full load to no load under such constant con-., 
ditions of operation as give the required full-load values, - 
the condition of full load being considered in all cases as 
the normal condition of operation. 



CONSTANT POTENTIAL DYNAMOS. 1 17 

The regulation of an apparatus intended for the gene- 
ration of a potential, current, speed, etc., varying in a defi- 
nite manner between full load and no load, is to be 
measured by the maximum variation of potential, current, 
speed, etc., from the satisfied condition, under such con- 
stant conditions of operation as give the required full-load 
values. 

If the manner in which the variation in potential, cur- 
rent, speed, etc., between full load and no load is not speci- 
fied, it should be assumed to be a simple linear relation ; 
i. e., undergoing uniform variation between full load and no 
load. 

The regulation of an apparatus may, therefore, differ 
according to its qualification for use. Thus the regulation 
of a compound-wound generator specified as a constant- 
potential generator will be different from that it possesses 
when specified as an over-compounded generator. 

The regulation is given in percentage of the full-load 
value of potential, current, speed, etc. ; and the apparatus 
should be steadily operated during the test under the same 
conditions as at full load. 

The regulation of generators is to be determined at con- 
stant speed. 

The regulation of a generator unit, consisting of a gen- 
erator united with a prime mover, should be determined at 
constant conditions of the prime mover ; i. e., constant 
steam pressure, head, etc. It would include the inherent 
speed variations of the prime mover. For this reason the 
regulation of a generator unit is to be distinguished from 
the regulation of either the prime mover or of the gene- 
rator contained in it, when taken separately. 

In commutating machines as direct current generators 



Il8 DYNAMO ELECTRIC MACHINERY. 

and motors, the regulation is to be determined under the 
following conditions : 

a. At constant excitation in separately excited fields, v 

b. With constant resistance in shunt-field circuits, and 

c. With constant resistance shunting series fields ; i.e., 
the field adjustment should remain constant, and should be 
so chosen as to give the required full-load voltage at full- 
load current. 

In constant potential machines the regulation is the 
ratio of the maximum difference of terminal voltage from 
the rated full-load value (occurring within the range from 
full-load to open circuit), to the full-load terminal voltage. 

In constant current machines the regulation is the ratio 
of the maximum difference of current from the rated full- 
load value (occurring within the range from full load to 
short circuit), to the full-load current. 

In over-compounded machines, the regulation is the 
ratio of the maximum difference in voltage from a straight 
line connecting the no-load and full-load values of terminal 
voltage as function of the current, to the full-load terminal 
voltage. 

77. Direct Driven Light Generators. — The tendency of 
modern engineering practice is to install lighting gene-_ 
rators which are directly connected with the steam engines 1 
which drive them. Owing to the inherent speed of engines 
being smaller than that of generators, direct connected » 
armatures are designed to run at a lower speed than belt- - 
driven ones. Economical construction demands that they 
be of the multipolar type. They require less floor space 
per kilowatt than the belt-driven machines ; and this is a • 
question of considerable importance in many installations. . 



CONSTANT POTENTIAL DYNAMOS. 



119 



They have a higher efficiency of operation consequent 
upon the elimination of losses in belting and counter- 
shafting. They also permit of operation of isolated plants 
in residences and other places where the noise resulting 
from belt-driven machinery would not be tolerated. 

In order that standard generators may be easily con- 
nected with engines of any make, and vice versa, commit- 
tees from the American Societies of Electrical Engineers 
and of Mechanical Engineers have recommended the 
adoption of the following standard sizes, speeds, and arma- 
ture shaft fits : — 



Sizes in K. W. Capacity . 
Speeds in Rev. per Minute 
Armature Fit in Inches . 




5 

45° 

3 


7-5 

425 

3 


10 
400 
3Y2 


375 
3Y2 


20 

35o 

4 


25 

325 

4 


35 
3 J ° 

4% 


Sizes in K. W. Capacity . 
Speeds in Rev. per Min. . 
Armature Fit in Inches . 


5° 
290 

5 


75 

275 

6 


100 

250 

7 


I2 5 
2 35 

7V 2 


T 5° 

220 

8 


200 

200 

9 


250 

190 

10 


300 

180 

n 



Fig. 91 shows a machine made by the Westinghouse 
Electric Manufacturing Company in standard sizes of 100, 
150, 200, 500, and 675 k. w., at 125 volts. The field 
frame is circular and divided in a vertical plane. The pole 
pieces are of laminated sheet steel, cast into the frame. 
Projecting from the field frame are brackets, which hold 
and carry the brush-holder mechanism. This consists of 
a ring concentric with the axis of the armature. Upon 
its rim is a gear, which engages with a worm operated by 
a hand-wheel. The simultaneous shifting of the brush 
can be accomplished by the turning of the hand-wheel. 
The slotted armature disks are made of sheet steel, and 



120 



DYNAMO ELECTRIC MACHINERY. 



are held together by cast-iron end plates. The disks and 
end plates are mounted upon a cast-iron spider, which also 
carries the commutator. The spider is fitted so as to be 
pressed upon the engine shaft and keyed to it. The con- 




Fig. 91. 



ductors are bars of copper, which are forged into shape on 
cast-iron formers wound and insulated with mica and ful- 
lerboard. 

Figs. 92 and 93 represent a front and rear view of a 



CONSTANT POTENTIAL DYNAMOS. 



21 



General Electric Company's Form L generator. The 
frame, of a circular form, is divided in a horizontal plane, 
and is made of soft cast iron. To it are bolted pole pieces 




Fig. 92. 

which are made of soft cast steel. A skeleton, circular, 
disk-like brush-holder yoke is fastened to the frame by 
means of three slots and bolts, and is capable of sufficient 
angular rotation to permit of the proper adjustment of 



122 



DYNAMO ELECTRIC MACHINERY. 



the brushes. The movement is accomplished by means of 
a hand-wheel and pinion. The armature spider is so con- 
structed that it receives the commutator as well as the 




Fig. 93- 



disks and the armature windings. It is open so as to offer 
no obstruction to the free and thorough circulation of air 
through it, which permits of a perfect ventilation. The 
windings are of copper bars, and the end connections are 



CONSTANT POTENTIAL DYNAMOS. 



23 



supported by flanges which protect them from mechanical 
injury. The commutator shell is pressed upon the arma- 
ture spider. 




Fig. 94. 



The Crocker Wheeler Electric Company's direct-con- 
nected and belt-driven generators differ from others which 
have been described, chiefly because of the shape of the 
field-magnet frame and the method of armature winding. 
The field frame shown in Fig. 94 is circular in form, and is 



124 



DYNAMO ELECTRIC MACHINERY. 



divided in a horizontal plane. These frames are of cast 
iron, and have short internal flanges on each side, which 
mechanically strengthen the frame, and offer considerable 
protection from mechanical injury to the field coils. The 
round poles are of cast steel, cast-welded into the frame. 
They are provided with removable cast-iron shoes, which 
are clamped in place after the field coils have been put on. 
The armatures, instead of being bar-wound, are wound 




with solid copper wire of large sizes, which are triple 
cotton covered. The conductors are threaded through 
tubes which are placed one upon the other, and which are 
made of micanite cloth and press-board rolled up on a 
form and glued together. The brush holders and brush 
rigging were shown in Figs. 48 and 49. 

The Sprague Electric Company manufactures two types 
of Lundell generators, both for direct connection and for 
belt connection. They are, namely, the split-pole type, 
which employs the principle laid down in paragraph 55 



CONSTANT POTENTIAL DYNAMOS. 



25 



for compensating for armature reaction, and the single-coil 
type, which takes its name from the peculiar shape of the 
field frame and poles, which permits of the use of but a 
single field coil. Both frames are of the circular type, 




the split-pole field being divided in a horizontal plane, 
and the single-coil type being divided in a vertical plane 
which is perpendicular to the axis of the armature. A 
split pole, with its windings, is shown in Fig. 95. The 
compound coil is placed nearer the shoe than the shunt 



126 



DYNAMO ELECTRIC MACHINERY. 



coil, and both are kept in place by lugs, as shown in the 
figure. Fig. 96 shows a 6-pole, single-coil type field- 




magnet frame with its coil inclosed in the frame. The 
brush holders which are employed on both types of ma- 
chine are illustrated in Fig. 97, the brushes being of 



CONSTANT POTENTIAL DYNAMOS. 



127 



carbon used radially, and being perforated to receive a bolt 
for clamping them to the holders. 

The Bullock Electric Manufacturing Company's direct 
connected generator, Fig. 98, has an external appearance 
similar to that of the generators of other companies. It 




Fig. 98. 

is different from them, however, in having peculiarly con- 
structed poles. These poles are made up of laminated 
steel stampings, which are much thinner than are ordi- 
narily used, and which have the peculiar shape shown in 
Fig. 99. In assembling these stampings to form the pole, 
every alternate one is reversed from the position which is 



128 



DYNAMO ELECTRIC MACHINERY. 



indicated in the figure. The method of assembling is 
shown in Fig. ioo. After assembling, it will be seen that 
the face of the pole for a short depth contains but one-half 
as much iron as the main body of the pole. This results, 
under normal excitation, in a saturated pole face. It has 
the same effect in preventing distortion of the field under 
the influence of armature reaction, as saturation of the 
teeth of the armature core. The teeth can, therefore, be 




Fig. 99. 



Fig. 100. 



operated at a smaller magnetic flux density. The hystere- 
sis losses in the teeth can accordingly be made smaller. 
The thinness of the stampings, and the ideally perfect 
lamination of the pole face, permit the use of a smaller 
ratio of tooth width to slot width, without the excessive 
eddy current loss in the pole face which would occur in 
ordinary machines. The possibility of using narrow teeth 
results in a reduction of the inductances of the armature 
coils. This facilitates effective commutation. 



CONSTANT CURRENT DYNAMOS. 



129 



CHAPTER VIII. 



CONSTANT CURRENT DYNAMOS. 



78. Direct Current Arc Lighting Generators. — For 

lighting by arc lights where considerable energy is ex- 
pended at the points of illumination, and where these 
points are separated from each other by considerable dis- 
tances, it is sometimes economical and desirable to connect 
the lamps in series and use a constant current. A single 
line then completes a circuit of all the lamps (Fig. 101). 
The line can be made 
of much smaller wire 
than in the case of a 
constant pressure cir- 
cuit, for on a constant 
current circuit as the 
load increases the power 
or energy transmitted is 
increased by raising the* 

potential, the current remaining unaltered ; while in a con- 
stant pressure circuit an increase of load is met by an 
increase of current, and the wires of the line have to be of 
sufficient size to safely carry the maximum. The size of - 
wir e necessary is dictated, not by the energy transmitted,- 
but by the current flowing, hence a wire large enough to 
supply just one lamp of a constant pressure circuit can- 
supply all the lamps of a constant current circuit. 



>o 



Fig. 101. 



130 



DYNAMO ELECTRIC MACHINERY. 



The more general forms of arc lamps have what is 
termed a spherical candle-power of 800, 1200, or 2000. 
Lamps used in search-lights and in light-houses often ex- 
ceed this in candle-power, and may consume many more 
amperes. The arc lamp of 800 candle-power takes a cur- 
rent of 4.5 amperes, that of 1200 candle-power 6.6 to 
6.8 amperes, and that of 2000 candle-power 9.6 to 9.8 
amperes. 

An ordinary arc lamp, as it is trimmed and adjusted for 
general use, requires between 45 and 50 volts to force its 
rated current through it. A generator supplying a circuit 
of say 2000 candle-power lamps with n such lamps in the 
circuit must be capable of generating a constant current of 
9.8 amperes. It must be able to regulate its pressure 
between the limits of 50 and 507/ volts. This is necessary 
in order that it may operate all the lamps or any part of 
the whole number. 

The current of an arc-light machine must not exceed 
nor fall below its normal value, no matter how suddenly 
the load is varied ; for the slightest change, even for a very 
brief instant of time, affects the quality of the light at the 
lamps. It is obvious that some mechanical device could be 
applied to an ordinary shunt -wound generator to cause it 
to give constant current, either by changing the position of 
the brushes or by varying the ampere turns of the field 
coils. However, any such device would be slow of opera- 
tion, and a sudden short circuit would cause a destructive 
current to flow before the regulator completed its action. 
It is, therefore, necessary to rely on the armature reactions 
for regulation, since they vary simultaneously with the cur- 
rent. All successful constant current machines are con- 
structed on this principle. The machine is designed with 



CONSTANT CURRENT DYNAMOS. 



131 



a field of very great magnetizing power, the armature re- 
actions are very great, and thus the total flux effective in « 
producing E.M.F. is reduced. A slight increase of current < 
in the armature materially increases the armature reactions. - 
The effective flux is thus reduced, and the pressure falls 
until the current returns to its normal value. Thus the 
machine is completely and instantly self -regulating. Since 
the field magnetization is kept constant and the machine 
produces constant current the field coils are series wound 
on all arc-light generators, and the cores of the field 
magnets are worked at a very high magnetic density, since 
the magnets are then more insensible to slight changes in 
the magnetizing force. In commercial machines the den- 
sities in the field cores are from 17,000 to 18,000 lines per 
square centimeter for wrought iron or steel, and from 9000 
to 1 1,000 lines for cast iron. 

In the armature high magnetic density is also required 
to prevent a sudden rise of voltage when the circuit is 
broken. With no current in the armature, the total mag- 
neto-motive force of the field magnets would be effective 
in producing E.M.F., and a destructive rise of pressure 
would result, since the total M.M.F. of the field magnets 
is much greater than the normal effective M.M.F. But a 
high magnetic density in the armature core leaves the latter 
incapable of receiving such an increase of flux, and there- 
fore destructive voltages are avoided. In practice the arma- 
ture core is designed to have a density of from 15,000 to 
20,000 lines per square centimeter at its minimum cross- 
section. 

A consideration of the foregoing theory of regulation 
shows that the following conditions should obtain more or 
less completely in a successful constant current generator : 



132 DYNAMO ELECTRIC MACHINERY. 

(a) since the current is small, there must be a great num- 
ber of armature turns ; (b) the magnetic field of the ma- 
chine must be much distorted ; (c) the path of the lines of 
force of the field coils must be long and of small area, so 
the M.M.F. cannot be readily changed ; (d) the path of the 
lines of force due to armature magnetization must be short 
and of great area, so the M.M.F. of the armature will change 
with the slightest change of current ; and (c) the pole piece 
must be worked at a high density. 

Evidently extreme difficulty is found in so designing the 
different parts of the machine as to give proper considera- 
tion to each of the conditions and yet produce a machine 
that will regulate for constant current at all loads. This 
leads to the introduction of automatic mechanical devices 
for aiding in the regulation. These devices must not be 
considered as being the sole regulators, for in every case 
they are secondary to the natural self-regulating tendency 
of the armature. In general they regulate for the gradual 
and greater changes of load, while the armature reactions 
take care of the smaller and more sudden fluctuations. 

There are two general systems of regulating arc dynamos. < 
The first method is to cause the machine to develop an 
E.M.F. in excess of that required for the load, and to then 
collect an E.M.F. just sufficient for the load in hand. This 
is done by shifting the brushes from the neutral plane (§50). 
In a closed-coil armature this causes a counter pressure to 
be developed in those conductors lying between the neutral 
and the commutating planes. This reduces the pressure 
to the desired amount. In an open-coil armature the ; » 
brushes, when in the maximum position, connect to the < 
circuit those coils of the armature which at that instant 
have the maximum E.M.F. generated in them. By shift- 



CONSTANT CURRENT DYNAMOS. 133 

ing the brushes either way coils can be connected to 
the circuit which have some E.M.F. less than the total 
E.M.F. generated in them, and the amount of shifting 
regulates the pressure on the line. 

The second method of arc-light dynamo regulation is to 
vary the magnetizing force in the field magnets just enough 
to put the required pressure on the line. Since the mag- 
netizing force is dependent on the ampere turns of the field 
coils, it can be varied either by cutting out or short circuit- 
ing some of the turns or by changing the current in them 
by means of a variable resistance which is shunted across 
the field terminals. In practice both these methods have 
been used. 

Whether regulation is effected by changing the position 
of the brushes, or by changing the field excitation, sparking 
will occur at the points of collection of the current if means 
are not provided to avoid it. Non-sparking collection could 
be obtained if the field were perfectly uniform all around 
the armature. In general this condition is impracticable, 
since it requires almost the whole armature to be covered 
by the pole faces, and it requires the density in the gap 
beneath them to be uniform. Considerations of magnetic 
leakage and armature reaction render almost impossible the 
satisfying of these conditions. Another and more prac- 
tical method is to employ for collection at one terminal of 
the machine two brushes connected in parallel. These are 
moved in opposite directions, thus giving the effect of a 
single brush of varying circumferential contact, whose 
center can always be kept in the neutral plane. This 
prevents bad sparking. The device is used quite success- 
fully in practice. There is, however, some question as to 
the advisability of resorting to it. 



134 



DYNAMO ELECTRIC MACHINERY. 



9. The Brush Machine. — Fig. 102 shows a standard 
160-light Brush arc generator, made by the General 
Electric Company. The armature revolves between the 



rdj rh 6 rh dfirt 
n ri/ri/n/A/ri/A 




Fig. 102. 



opposed pole faces of two sets of field magnets, __Like 
poles are opposed to each other. The flux, therefore, 
takes a path out of the opposing pole faces into the arma- 



CONSTANT CURRENT DYNAMOS. 



135 



ture core, and then circumferentially through the core and 
out into the next pair of opposing pole faces. 




Fig. 103. 



The armature, Fig. 103, consists of a number of coils or 
bobbins placed on a ring core of greater radial depth than 
breadth, and the pole faces cover the sides instead of the 




Fig. 104. 



circumference. The bobbins are protected by an insulat- 
ing box, shown in Figs. 104 and 105, but are not surrounded 
by any masses of metal. This fact, coupled with the fact 
that the armature is of such a shape as to cause great air 



136 



DYNAMO ELECTRIC MACHINERY. 



disturbance, insures exceptional ventilation of the armature, 
and tends to prevent the "roasting out " of the coils when 
subject to an overload. This machine is of relatively slow 
speed, the larger sizes running at only 500 R.P.M. 




Fig. 105. 



At a given instant of time, the different coils on the 
moving armature have E.M.F.' s of widely different magni- 




Fig. 106. 



tudes induced in them. The commutator, Fig. 106, is so 
designed that it connects the coils of highest E.M.F. in 
series with each other to the external circuit, and con- 
nects the coils of medium E.M.F. in multiple with each 



CONSTANT CURRENT DYNAMOS. 



137 



other to the external circuit, while those of smallest E.M.F. 
are cut out entirely from the circuit. 

The bearings are self-lubricated by means of rings. 
Since the poles are on the sides of the armature, side 




Fig. 107. 



play in the bearings must be prevented. To this end the 
commutator end of the shaft is turned with six thrust col- 
lars, as seen in Fig. 107, which are engaged by correspond- 
ing annular recesses in the brasses. 



38 



DYNAMO ELECTRIC MACHINERY. 



Regulation on these machines is effected by a variable 
resistance in shunt with the field coils; and as the field 
current is changed the position of the brushes is also 
changed, not to collect current at a lower voltage as de- 
scribed in § 78, but to obtain sparkless collection. These 
two operations are performed by a regulator (Fig. 108), 




Fig. 108. 

which is attached directly to the frame of the machine. 
The mechanism consists of a rotary oil-pump driven by a 
belt from the armature shaft, a balance valve of the piston 
type, and a rotary piston in a short cylinder, which is 
directly connected to an arm sweeping the contacts of the 
field-shunt rheostat. The valve is operated by a lever 



CONSTANT CURRENT DYNAMOS. 



139 



actuated by a controlling electro-magnet which is energized 
by the whole generator current. At normal current the 
valve is centrally placed, and the oil from the pump flows 
around the overlapping ports into the reservoir without 
effect (See Fig. 109). If the current rises above the nor- 
mal, the armature of the controlling magnet is attracted, 
the balance valve moves up, and oil enters the cylinder, 




Fig. 109. 

moving the rotary piston in a clockwise direction. The 
shaft of this piston moves the arm of the rheostat, cutting 
out resistance and thus lowering the field exciting current. 
At the same time a pinion on the shaft, seen in Fig. no, 
actuates a rocker arm which moves the brush holders to a 
position such that the collection by the brushes will be 
sparkless. When the current returns to normal the 
adjusting spring, seen in Fig. in, returns the lever and 
balance valve to the central position. If the current falls 



140 



DYNAMO ELECTRIC MACHINERY. 







Or-aln Valv* 



Fig. no. 




Fig. in. 






CONSTANT CURRENT DYNAMOS. 141 

below the normal, these operations are reversed. The 
tension of the adjusting spring can be regulated from the 
outside of the dust-proof case by a hard rubber knob. 
From the nature of the case the parts are always well 
lubricated. 

It is claimed for this regulator that it can bring the cur- 
rent back to normal from a dead short-circuit in from 31 to 
4 seconds. 

80. The Westinghouse Arc-Light Machine — Fig. 112 
shows a 7 5 -light direct current arc-lighting generator, 
made by the Westinghouse Electric and Manufacturing 
Company. It is of rigid construction, the bearing sup- 
ports being cast integral with the frame. For facilitating 
transportation and repairs the yoke parts in the middle 
on a horizontal plane. The bearings are of the self- 
oiling, self-aligning type described in § 41. The armature 
shown in Fig. 113 is of the open-coil type, which gives a 
unidirectional but not absolutely continuous current. The 
slight pulsations of the current thus set up, while not 
affecting the steady mean value of the current, cause a 
slight constant vibration in the mechanism of the lamps 
that helps overcome any tendency to stick or a failure to 
feed the carbons. A unique feature of this armature con- 
sists in its having two separate sets of windings on the 
same core, each set having its own commutator. The coils 
and commutators are so arranged that while a set of coils 
of one winding is being cut into or out of the curcuit a set 
of the other winding is supplying current to the line. It is 
claimed for this method of connecting open-coil armatures, 
that it yields a more satisfactory current, and obviates the 
vicious sparking at the commutator found in other types 



142 



DYNAMO ELECTRIC MACHINERY. 



of open-coil machines. The armature is made of lami- 
nated steel sheets punched with T-shaped teeth between 
the winding slots. The armature coils are wound on 




Fig. 112. 

molds, and insulated and mounted on the armature as 
shown in Fig. 114. They are held in place by wooden 
wedges forced into the loops left at the ends of the arma- 
ture. This construction admits of removing one coil for 
repairs without disturbing any of the other coils. 



. 



CONSTANT CURRENT DYNAMOS. 



143 



This machine differs from 
the general type of arc-light- 
ing machines in that it is 
separately excited, a small 
auxiliary machine generating 
current for the field coils at 
100 volts. This obviates the 
possibility of danger from a 
too high pressure resulting 
from an open circuit. 

Regulation is obtained by 
careful design, so that the 
armature reactions cause the 
voltage to vary in just the 
proper proportions, as de- 
scribed in § 78. The exciting 
field current is regulated to 
give the proper excitation by 
a series rheostat. By this 
means the line current can 
be raised or lowered slightly 
if desired, without affecting 
the self -regulation. 

Fig. 1 1 5 shows the double 
commutator of this machine. 
The segments are easily re- 
moved and replaced in case 
they wear or burn out. 




81. The Wood Arc Dynamo. — Fig. 116 shows a Wood 
constant current dynamo for lighting 125 2000 c.p. lamps. 
This machine claims an efficiency of 90 per cent on full 



144 



DYNAMO ELECTRIC MACHINERY. 



load. The bearings are self -oiling, and may be removed 
for repairs or inspection without removing the armature. 
The armature has large radiating surface and shallow wind- 




Fig. n 4 . 



ing, and its temperature does not rise more than 40 C. 
above the temperature of the room. This armature is of 
the closed-coil type, requiring a commutator of many seg- 
ments with but a small potential between any two adjacent 






CONSTANT CURRENT DYNAMOS. 



145 



ones. This fact, and the use of two brushes in parallel, as 
explained in § 78, obviates all sparking. 

This machine operates by generating full pressure at all 
times, and by automatically setting the brushes to take off 
just such potential as is necessary. This allows of regu- 
lation without making use of rheostats, separate ex- 
citers, wall controllers, motors, or relays. The regulating 




Fig. 115. 



mechanism is set in operation by a sensitive and rather 
powerful electro-magnet excited by the main armature 
current. This attracts a lever which is restrained by an 
adjustable coiled spring. A variation in the current 
strength causes this lever to throw into train one or the 
other of two oppositely revolving fiber friction cones, which, 
acting through gears and levers, shifts the brushes the re- 



146 



DYNAMO ELECTRIC MACHINERY. 



quisite amount, and also varies their angular contact or 
collecting extent. All the delicate parts of this mechanism 
are inclosed in the pillar supporting the commutator end 
of the armature shaft, and are thereby protected from 




Fig. 116. 



injury, dust, and grit. The wearing surfaces of this regu- 
lator are all large and the speed is slow, so that wear is re- 
duced to a minimum. Without any change of adjustments 
this regulator will operate when run either way, which is 
an advantage when two or more dynamos are run from one 
engine, and economy of space is essential, or in case of 
accident to a prime mover. 



CONSTANT CURRENT DYNAMOS. 



147 



82. The Excelsior Arc Dynamo. — This machine, Fig. 
117, is a closed-coil ring armature generator, having pole 
faces that cover both the sides and the circumference of 
the armature. The interesting feature of this machine is 
the method of regulation. The proper potential is sup- 
plied to the line by using both methods of control in con- 
junction ; that is, sections of the field windings are cut in 
or out of circuit, and at 
the same time the posi- 
tion of the brushes is 
shifted. The proper 
motion of the field regu- 
lator arm and of the 
brush holder is obtained 
by means of a small 
motor whose field is 
"sneaked" from the 
main magnets of the 
machine. This motor 
is operated by a device 
shown in Fig. 118. The 
whole device is inserted 
in series with one of the 
mains from the gener- 
ator. The right-hand 
lever is of insulating material, with the contact blocks 
a and b properly placed upon it. The left-hand lever 
is of conducting material, and is capable of being attracted 
by the electro-magnet which is excited by the main 
current. The magnet and spring are so adjusted that 
when the normal current is flowing, both a and b are 
in contact with the left lever, and the current flows in the 




Fig. 117. 



148 



DYNAMO ELECTRIC MACHINERY. 



three shunt paths, R, R v and R 2 . There will be no cur- 
rent in the armature of the regulating motor, since the 
potential at brush x is equal to the potential at brush y. 
If now the line current becomes too strong the magnet 
attracts the left lever to it and the contact at a is broken. 







a 


b 


j 




k^ 






, | 






L 


- ] 




,-fi 


i\ 




l r 






1 










/ 


\ 






1 




t~ 


















N 










> 
























+• 1 Motor 


M» 












< 










>/? 










s 














>«1 




>/?2 < 


















7b t/ne 
















from Dynamo 



Fig. n8. 



Immediately the current flowing through b divides at the 
brushy, part going through R 2 and part through the motor 
armature and R v The motor will then revolve in a given 
direction, and by simple mechanical devices will cut out 
sections of the field windings, and will shift the brushes 
until the normal current is flowing, when contact is again 



CONSTANT CURRENT DYNAMOS. 149 

made at a and the controlling motor stops. If the line 
current drops below normal, the spring pulls the lever 
away from the magnet and the contact at b is broken. 
Part of the current then flows from y to x through the 
motor armature. It therefore revolves in a direction op- 
posite to that which it had before. The brushes on the 
dynamo are shifted back again, and more sections of field 
winding are put into circuit. 

In practice the levers and the magnet are mounted on 
the wall or the switch-board, while the regulating motor is 
mounted on the dynamo frame. 

When the current is broken at a or b, there is no serious 
sparking, since there are always two circuits in shunt with 
the break. The whole current of the dynamo does not 
exceed ten amperes ; and the resistances R, R v and R 2 are 
so proportioned that only a small portion of that flows 
through a or b. 

83. The Ball Arc Generator. — Fig. 119 shows a double 
armature, automatic regulating constant current generator, 
made by the Ball Electric Company. Two independent 
circuits, each automatically controlled, can be operated 
from the one machine, since it has two distinct armatures, 
commutators, and regulators. The advantage claimed for 
this arrangement is that the pressure has to be but half as 
high as if the two circuits were united and fed by a single 
armature. Yet if it be undesirable to bring the ends of 
two circuits into the power-house, they can be connected 
in series, and fed by the two armatures also connected in 
series, and then the voltage per armature will be half that 
of a single armature machine giving like results. 

The armatures are of the closed-coil ring type. The air 



150 DYNAMO ELECTRIC MACHINERY. 




CONSTANT CURRENT DYNAMOS. 151 

gap between pole faces and armature is short in length and 
great in area, requiring a minimum of magnetic excitation. 
The commutator is built up of a great number of segments, 
the potential between any two adjacent segments not ex- 
ceeding fifteen volts. This assures sparkless commutation. 

This generator is regulated by shifting the brushes until 
pressure of a suitable magnitude is collected. 

A magnetic body placed in a magnetic field will tend to 
rotate until the longest axis is parallel to the magnetic lines 
of force. This principle is applied to the Ball regulator as 
follows : A magnetic portion of the brush carrier is made a 
part of the magnetic circuit, and is placed in a recess of the 
dynamo frame. It tends to assume an axial position with 
a force varying as the flux through it. As the line current 
increases the flux increases, and the brush holder, which is 
mounted on ball bearings, rotates, shifting the brushes the 
required amount. The impulse to regulate is applied 
directly to the brush holder, instead of being communi- 
cated to it by by a more or less complex mechanism. The 
magnetic tendency to shift the brush holder is opposed by 
gravity. 

84. The Thomson-Houston Dynamo. — The Thomson- 
Houston arc generator is of a type entirely different from 
the other machines here described, not only in appearance, 
but also in method of armature winding and of regulation. 
A view of this machine is given in Fig. 1 20. Each field 
coil has for its core an iron tube, flanged exteriorly at each 
end to form a recess for the windings, and fitted at the 
armature end with a concave iron piece that surrounds part 
of the armature. This tube, with the flanges and the cup- 
shaped end, is cast in one piece. The farthermost flange 



152 DYNAMO ELECTRIC MACHINERY. 

of each field core is bolted to a number of wrought-iron 
connecting-rods which hold the magnets in place, protect 
the field windings, and take the place of the yoke of other 
machines in completing the magnetic circuit. The mag- 
nets are mounted on a frame, including legs and bearing 
supports for the armature shaft. 

The armatures of the older machines of this type are 
spheroidal in shape, while the more recent ones have ring 
armatures which are more readily repaired or rewound. 
The winding of either form of armature is peculiar in that 
only three coils are employed, set with an angular dis- 
placement from one another of 120 degrees. In the ring 
armature no difficulty is found in properly winding these 
coils ; but in the old spherical armature the following de- 
vice was employed to secure the windings, and give each 
of them the same average distance from the pole face. A 
hollow spheroidal iron core was keyed on the shaft. The 
core had three rows of externally projecting wooden pins. 
Between these pins the coils were wound, half of coil A 
being wound first, then at 120 degrees distance half of coil 
B was wound, covering parts of coil A. Then at 120 de- 
grees from both A and B all of coil C was wound. Over 
this, but in its proper angular position, the other half of 
coil B was wound, and finally the rest of coil A was put in 
place. By this arrangement the average distance of each 
coil from the pole face or from the iron core was the same. 
In either type of armature the inner ends of the three coils 
are joined to each other, and are not attached to any other 
conductor, an arrangement unique in direct current dyna- 
mos. The outside ends are connected to the segments of 
a three-bar commutator, from which the current is collected 
by four copper brushes connected in multiple. 



CONSTANT CURRENT DYNAMOS. 



153 



Regulation is obtained by shifting the brushes in the 
following manner. Fig. 1 2 1 shows the two possible rela- 




Fig. 120. 



tions between brushes and commutator that may exist at 
any instant. Both brushes of each set may rest on one 
commutator bar, or the brushes of one set may span the 



154 



DYNAMO ELECTRIC MACHINERY. 



break between two of the bars. These conditions are re- 
peated three times at each brush for each revolution. If 
the dotted line shows the position where the maximum 
E.M.F. is generated in the coils, then in Fig. \2\a the 
two most active coils are connected in series with the out- 
side circuit, while the coil near the position of least activity 
is out of circuit. In Fig. \2\b the two less active coils are 
in multiple with themselves and in series with the most 
active coil and the external circuit. In practice the 




Fig. i2i 



brushes of a set are 60 degrees apart, leaving 120 degrees 
between the leading brush of one set and the following 
brush of the other set ; and since 1 20 degrees is the angular 
measure of the length of a commutator bar, there is no 
coil out of circuit at normal load, two being always in 
parallel and in series with a third. If the current rise 
above the normal the leading brushes move a small angle 
forward, while the following brushes recede through three 
times that angle. This will shorten the time that a single 
coil gives its whole E.M.F. to the circuit, and will place it 
more quickly in parallel with a comparatively inactive coil. 
But such a movement will reduce the angular distance be. 



CONSTANT CURRENT DYNAMOS. 



155 



tween tne nearest brushes of the opposite sets to less than 
120 degrees, hence the machine will be short circuited six 
times per revolution, since one brush of each set will touch 
one segment of the commutator at the same time. If the 
current in the line falls below normal, then the brushes 
close together, and the time that a coil is in series is 
lengthened, and the time that it is in parallel with an 
inactive one is lessened. 



1, ,1 



& 







^AMM/J 




Field 



Coils 
Fig. 122. 



The arrangement for moving the brushes is shown in 
Fig. 122. The leading brushes are shifted forward on an 
increase of current merely to help avoid sparking. The 
brushes are moved by levers actuated by a series magnet 
A. This magnet is normally short circuited by the by- 
pass circuit. On an undue rise of current this circuit is 
broken by the series magnet B. A then becomes more 
powerful, and the levers separate the brushes. While the 
machine is in operation the circuit -breaker C is constantly 
vibrating, and brushes adjusting to suit the load. A high 



156 DYNAMO ELECTRIC MACHINERY. 

carbon resistance is shunted across C to prevent sparking 
at that point. 

As might be expected, with but three parts to the com- 
mutator and collection made with small regard to the 
neutral point, the sparking of this machine is such as to 
speedily ruin the commutator and the brushes, if means 
are not taken to suppress it. A rotary blower is mounted 
on the shaft, and is arranged to give intermittent puffs of 
air, which at the right moment blow out the spark. The 
insulation between the segments is air, considerable gap 
being left between them, and through these gaps the 
sparks are blown. 

85. Western Electric Arc Dynamo. — Fig. 123 represents 
this machine, which is regulated by means of shifting the 
brushes. The brush and rocker are connected by means 
of a link and a ball and socket joint with a long screw. 
This screw is held in position by a nut. When the current 
is normal, both the nut and screw revolve at the same 
rate, and consequently there is no end movement of the 
screw. The brush, therefore, remains stationary. An 
electro-magnet, energized by a coil which is in series with 
the main circuit, attracts an armature whose movement 
towards the magnet is opposed by the action of a spring 
which is susceptible of regulation. When the current has 
too high a value, the electro-magnet attracts its armature 
more strongly than ordinarily. The latter moves toward 
the magnet, and by its movement catches a stop on the re- 
volving nut, and thereby prevents the revolution of the nut 
until the resulting longitudinal movement of the screw has 
shifted the brushes sufficiently to bring the current to its 
normal value. If the current be too weak, the spring 
which is attached to the magnet armature overpowers the 



CONSTANT CURRENT DYNAMOS. 



157 



electro-magnetic attraction. The resulting movement of 
the armature stops the rotation of the screw and permits 
the rotation of the nut. This results in a longitudinal 
movement of the screw and a shifting of the brushes in 
the opposite direction. The stopping and starting of the 
nut and screw is accomplished through the medium of 




Fig. 123. 



small triggers controlled by the armature of the series 
magnet. The triggers are fastened to a gear rotated from 
the main shaft by a belt. They engage with stops on the 
nut and screw respectively. 

Fig. 1 24 gives a sectional view of the regulator, and the 



158 DYNAMO ELECTRIC MACHINERY. 




Fig. 124. 



CONSTANT CURRENT DYNANOS. 



159 




160 DYNAMO ELECTRIC MACHINERY. 

trigger which engages with the screw is shown at «, and 
the one which engages with the nut is shown at m. 

Fig. 125 represents the details of the armature con- 
struction. The latter is ring wound with a large number 
of turns in each section. 



MOTORS. 161 



CHAPTER IX. 

MOTORS. 

86. Principle of the Motor Any direct current dy- 
namo will act as a motor if supplied with current from some 
external source. This source may be a constant E.M.F. 
system, a constant current system, or any other system. 
The rotation of the armature is a direct consequence of the 
conditions laid down in § 20.. It is evident that if the 
negative and positive terminals of a dynamo be connected 
with the corresponding terminals of some external source 
of supply, the direction of flow in the armature will be- 
reversed. Irrespective of the multipolarity of the field or 
of the method of armature winding, the electro-dynamic 
actions between the field and all the currents in the in- 
ductors will conspire to produce rotation in one direction. 

87. Direction of Rotation To determine the direction 

of movement of an inductor carrying a current of known 
direction in a magnetic field of known direction, one may 
employ a modification of Fleming's rule. Thus in a dynamo 
the thumb and two first fingers of the right hand deter- 
mine the direction of induced E.M.F. as shown in Fig. 
1 26. But in a motor the thumb and two first fingers of the 
left hand can be made to determine the direction of motion 
as shown in Fig. 127. 

If in a dynamo the direction of the field flux be not 



1 62 



DYNAMO ELECTRIC MACHINERY. 



altered, and if the armature be supplied with a current flow- 
ing in the same direction as when the machine was operated , 
as a dynamo, the direction of rotation will be reversed. )f &?<- , 
Thus, if the positive brush of a dynamo be connected to 
the positive terminal of an external source of supply, and if 
the negative brush be connected to the negative terminal, 
then the direction of current flow in the armature will be 
reversed.* The direction of rotation of the armature, in* 



D »*crio* 




HELD MAGNET 
HOBTM POUE. 

I DYNAMO •. RIGHT HAND* 




Field magnet, 

north pole. 

MOTOR LEFT HANO.v 



Fig. 126. 



Fig. 127. 



series-wound machines, since the field flux has its direction 1 
also changed, will be reversed In shun ^wound, separately 
excited, and magneto machines, since these do not have 
their fields reversed, the direction of rotation will be un-_ 
altered. Compound-wound machines will have the same 
or reversed direction of rotation, depending upon whether 
the magnetizing effect of the shunt coils is stronger or 
weaker than that of the series coils. In a compound gen- 
erator the actions of the shunt coils and the series coils are 
cumulative, i. e., in the same direction ; but when used as a 
motor the actions are differential, i. e., opposed to each other. 
Motors are also wound so as to have cumulatively acting 
series coils. 



MOTORS. 163 

88. Speed Conditions If an electric motor be supplied 

with electrical energy, it will vary its rate of rotation until 
it has attained such a speed as will produce an equality be- 
tween the input of energy and the output of energy. The 
latter appears both as useful work and as losses. In the 
case of a motor, speed acts toward electrical energy like 
temperature in the case of heat energy. Temperature 
always rises until the heat energy which is produced is 
equal to the heat energy which is disposed of by con- 
duction, convection, and radiation. 

The electrical energy which is communicated to a motor 
is transformed, a, into useful mechanical energy, which is 
taken from the armature shaft either by a belt or by direct 
connection ; b, into friction at the bearings and at the 
brushes; c, into windage; d, into foucault and eddy currents; 
and finally e, into ohmic heat energy in the motor's electrical 
circuits. The energy required per unit of time to overcome 
friction, windage, hysteresis, and foucault and eddy currents 
increases as the speed of rotation increases. Nearly all 
practical loads put upon a motor — machinery in one form 
or another — require an increase of power for an increase 
of speed. Therefore, if a given amount of electrical power 
be communicated to a motor under load, the armature will 
assume some speed of rotation, so that a balance between 
the input and the output of energy is maintained. 

89. Counter E.M.F. — If the variation of losses and useful 
energy with the speed were the only conditions governing 
the speed, then there would result in practice variations of 
speed through enormous ranges. But there is another con- 
dition affecting the speed. The armature, by varying its 
speed, not only governs the rate of expenditure of energy, 
but also governs the amount of electrical energy received. 



1 64 DYNAMO ELECTRIC MACHINERY. 

The armature of a motor revolving in a field under the 
influence of supplied electrical energy differs in no respect 
from the same armature revolving in a field under the in- 
fluence of supplied mechanical energy. There is an E.M.F. 
generated in it which is determined by the speed and quan- 
tity of flux. For the same speed and the same flux there 
would be generated the same E.M.F. in the case of a motor 
as in the case of a dynamo. The direction of this E.M.F. 
is, however, such as to tend to send a current in a direction 
opposite to that of the current flowing under the influence 
of the external supply of E.M.F., according to §87. 
Therefore this pressure which is induced in the armature of 
a motor is called counter electro-motive force. The current 
which will flow through the inductors of an armature is 
therefore equal to the difference between the supply E.M.F. 
and the counter E.M.F. divided by the resistance of the 
armature, or 



Ra 

For example, an unloaded 1 k.w. shunt motor having an 
armature resistance of 1 ohm, when connected to a con- 
stant source of potential supply of 100 volts, would not take 
a current of 100 amperes as dictated by Ohm's law, unless 
its armature were clamped so as to prevent rotation. If 
undamped, its armature would assume such a speed that it 
would have induced in it a counter E.M.F. of say 97.5 
volts. The current then flowing in the armature would be 

100 — 07. c 

±i_?J = 2.5 amperes. 

The power represented by this current, viz., 2.5 X 100 
watts, would all be expended in overcoming the losses of 
the machine. 






MOTORS. 



I6 5 



90. Armature Reactions Since in a motor, for a given 

direction of rotation and flux, the current in the armature 
flows in a direction contrary to that * 
which it would have as a dynamo, 
therefore the effect of the motor 
armature cross turns is to skew the 
field against the direction of rotation, - 
as in Fig. 128. Tnis increases the 
magnetic density in the leading pole 
tip, and decreases it in the trailing 
tip. This necessitates, for sparkless / 
operation, a backward lead, or a lag, 
of the brushes. If the brushes were 
in the same place as when the ma- 
chine was operated as a generator, 
the direction of armature current 
having been reversed, then the de- 



magnetizing or back turns of the 




Fig. 128. 



generator would become magnetiz- - 

ing turns for the motor ; but with the brushes shifted to 
a position of lag, then the motor has also demagnetizing 
or back turns. 

91. Efficiency — In a compound- wound motor connected 
to a constant p ressure supply, 

Let R sh = resistance of shunt field coils, 

J?a+s= resistance of armature plus that of the series field 
coils, 
V= number of revolutions per minute, 
T = torque given off at the pulley in pound feet, 
E = supply voltage, 

I n = no-load armature current in amperes, and 
/ = armature current when torque T is yielded. 



166 DYNAMO ELECTRIC MACHINERY. 

,_. „ . useful power output 

The efficiency = —. : — f * , 

electrical power input 

hence, 

_2^rFTjMh- 
e E 2 ' 

A-sh 

The useful power can be expressed as the difference 
between the power input and the losses. Now at no load,- 
when there is no useful power output, the following rela- 
tions exist : ^ 

EI n + — - = power input, 

and 

I 2 R a+s = P r = power in the armature and series coils. 

Assuming the friction, the windage, the foucault current, 
and the hysteresis losses to be constant and the same as 
at no load, we have for their value a constant loss = the 
no-load power input — the variable loss* 
Hence, 

The constant loss = P f = EI n + P sh - P' ', 
where ^ 

P sh = loss in shunt coils = — — 

■Rsh 

The efficiency under load will therefore be , 
EI-P f -I 2 P a+s 







EI + P sh 




In a 


shunt motor R a + S represents the armature 


resistance 


only 


; hence, 


ei— p f — rp a 






e — 


EI+P s n 




In a 


series motor P sh 


= o ; hence, 






e = 


Ei-p f -rp a+s 





EI 






MOTORS. 



167 



In the first of these three expressions for efficiency, 
solving for that value of / which will give a maximum effi- 
ciency, we have 

de {EI + P sh ) (E - 2 IE a + s ) - (EI-P f ~rX a+s )E 



dl 

whence 



EI+P S} 



s/ 



Ef ~f~ E sh P s) 



R n ^ "*" E* E 



Fig. 129 gives a set of curves indicating the perform- 
ance of a motor whose fixed losses are large. Fig. 130 




Fixed/Losses in Shunt Coils, friction, foucault, and hysteresis. 



Power Input 

Fig. 129. 

gives a set of curves for an exactly similar machine save 
that the fixed losses are smaller. They might be con- 
sidered as taken from the same machine as the first, but 
with journals better oiled, and hence with less friction loss. 
The difference in the efficiency curves is noticeable. 



i68 



DYNAMO ELECTRIC MACHINERY. 



Abscissae in all cases represent power input. The ordi- 
nate of P at any given load shows the power input at that* 
load. The constant ordinate of F represents the power 
consumed by the fixed losses,"' which is constant for all 
loads. The ordinates of V, measured from F, follow the 




Fixed Losses in Friction, foucault, hysteresis and shunt coils. 



Power Input 

Fig. 130. 

law PR a+si and represent the variable loss at various loads. 

Therefore the total loss for any load is represented by the 

total ordinate of Fat that load. The difference between 

the power input and the losses* gives the useful power, 

which is represented by the difference of the ordinates of . 

™ , -rr ^ , r 1 • useful power , , 

P and V. The values of the ratio %• for each 

power input . 

load are plotted in the efficiency curve. Comparing the 

curves of the two machines, it is seen that to get a high 

efficiency at full load the variable loss must be kept small, 

while to obtain a good efficiency at small loads, the fixed 






MOTORS. 



169 



losses must be made small. The shape of the efficiency 
curve can be controlled by a proper adjustment of the 
relation which exists between the fixed and the variable 
losses. 

92. Starting Rheostats When the armature of a motor 

is at rest there is no counter E.M.F. ; and at the instant of 
closing the circuit a destructive current would flow if a re- 
sistance were not first inserted in the circuit, except in the 
case of very small motors whose armatures have small 
moments of inertia. As the speed increases the resistance 
can be lessened without allowing too severe a current to 
flow, and when full speed is obtained the resistance must 
all be cut out to avoid loss. In order that counter E.M.F. 
may be generated from the start, the shunt field circuit 
must first of all be closed. These ends are obtained by the 
use of a starting-box or rlieostat, the wiring of the ordinary 
type of which is shown in Fig. 
131. Its main feature is a con- 
tact arm pivoted at its center, 
and revolving through almost 
180 , making various contacts. 
This arm is connected to one 
terminal of the supply as shown. 
As it is slowly turned on, one 
end of it first makes a connection 
which completes the shunt field 
circuit. Then the other end 
makes a contact which closes 
the armature and series coils 
through the maximum resistance 
of the starting-box. As the 
speed increases, the revolving p^ I3I# 




170 



DYNAMO ELECTRIC MACHINERY. 



arm is made to cut out the resistance, piece by piece, until 
it is finally all out of the circuit and the machine is run- 
ning independent of the starting rheostat. 

Fig. 132 shows such a starting-box as made by the 
Crocker Wheeler Company. The brass contact points 

and the arm are mounted 
on a slate slab, which 
serves as the top of an 
open-work cast-iron box 
which contains the resist- 
ances in the form of spiral 
coils of bare wire. The 
wire is generally either of 
German silver or of some 
one of the special iron 
alloy resistance materials. 
A shunt motor may 
have its armature coils 
destroyed by an excessive 
rush of current resulting from a dropping or ceasing alto- 
gether of the supply voltage followed by a sudden renewal 
after the speed of the armature has fallen. These condi- 
tions may arise through accidents to mains or because of a 
too heavy load on mains of insufficient cross-section. An 
armature may also be burned out by an excessive cur- 
rent due to overloading the motor. The resulting lower- 
ing of its speed is accompanied by a corresponding lowering 
of the counter E.M.F. Again, a too high supply voltage, 
which might result from some cross or other accident might 
cause a destructive rush of current. To meet these condi- 
tions, starting rheostats are often made with attachments 
for opening the circuit on no voltage or low voltage, and 




Fig. 132. 






MOTORS. 



171 



Release Magnet 




Fig. 133. 




Fig. 134. 



172 



DYNAMO ELECTRIC MACHINERY. 



others with attachments for opening the circuit on overload. 
Some have both attachments, but it is modern practice to 
remove the overload attachment from the starting-box and 
put it on the switchboard. Fig. 133 is a diagram of the 




Armature 



Details of Release Magnet 




m 



\=r~ 



e e]H 



Fig. 135. 



wiring of a starting rheostat for a shunt motor with auto- 
matic release and low-voltage attachment. Fig. 134 gives 
a front view of this same instrument. When the starting- 



MOTORS. 



173 



handle is placed in the "on" position, the magnet in the 
field circuit holds it there, although a spring tends to throw 
it back. If now, because of low voltage, the current in field 
and magnet becomes weak, the magnet is no longer able to 
detain the handle, and the spring throws it to the " off " 
position, where it stays until the motor is again turned on 
by an attendant. 

Figs. 135 and 136 show a view and a diagram of the 
wiring of a rheostat with both release and overload attach- 




Fig. 136. 



ments. The former is similar to the one just described, 
while the overload attachment consists of a magnet in the 
armature circuit which on overload becomes strong enough 
to attract to itself a pivoted iron arm supplied at its end 
with a device which short circuits the field current around 



174 



DYNAMO ELECTRIC MACHINERY. 



the release magnet. This causes the latter to let the 
starting-handle drop as in the case of low voltage. 



93. Characteristic Curves of a Shunt Motor A shunt 

motor, having a small R a and a large R sh , and having the 
field well saturated, will give a fa_irly constant speed under 
all loads, if supplied from a constant pressure circuit. 
This is shown by the curves in Fig. 137, which are from 
a bipolar, shunt-wound, 10 horse-power, 230-volt Crocker, 

Wheeler motor. 
^C3 A shunt motor when 

90 h started cold on no load 

HI 

so £ quickly arrives at a speed 

70s which then gradually 

60 1 rises to a maximum. The- 

50 § gradual heating of the 

40 1 field coils increases their 

30 5 resistance. This allows 

u 

20 1 less current to flow in 

o 

10 H them, and the resulting 

magnetic flux is less. 

Therefore the armature 



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must rotate faster to gen- 



94. Compound-Wound Motors In silk-mills and other 

textile factories where any slight variation in the speed 
affects the character of the manufactured product, com- 
pound motors give a satisfactorily constant speed. The 
object of the compounding coils is to weaken the flux in 
the armature as the load increases. If, at a given load, 
under the influence of shunt excitation alone, the speed 



MOTORS. 



175 



would fall a certain per cent of the speed at no load, then 
the armature flux must be lessened by the same percentage 
in order to bring the speed up to its original value. In 
calculating the number of series turns, account must be 
taken of the fact of 
magnetic leakage, 
since the regulating 
coils are on the field 
magnets and not on the 
armature direct. Cu- 
mulatively compound- 
wound motors are used 
in order to obtain a 
large starting torque. 
The influence of the 

series coils is not very great after full speed has been 
attained. 





mmmu 

Shunt Field 



Fig. 138. 



95. Hand Speed Regulation A rheostat placed in the 

field circuit of a shunt motor can be used to vary the 
speed of the motor at will, as in Fig. 138. An increase 
of the resistance will decrease the current in the field 
coils. As a result the armature magnetic flux will decrease 
and hence the speed will increase. If the fields be pretty 
well saturated, it will require a resistance of some con- 
siderable size, say twice as large as the field resistance, to 
cut the current down enough to materially reduce the 
flux and increase the speed. Motors of older make seldom 
had fields magnetized anywhere near saturation. There- 
fore they are very susceptible to the slightest change of 
resistance in their field circuits. If the demagnetizing 
armature ampere turns be large, it is possible for a motor 



176 



DYNAMO ELECTRIC MACHINERY. 



T 



to increase its speed under increase of load. This is due 
to the decrease of armature flux. 

96. Speed Regulation by Series Resistances. The 

speed of a motor on a constant pressure circuit can easily 

be varied over a wide range, 

Series R.^ r r „ ° 

from rest to full speed, by 
manipulating a resistance 

• , in series with it. The use 

of this method is not to 
be advised save for ex- 
perimental purposes, since 
it is very wasteful of en- 
ergy. The I 2 R loss in 
the regulating resistance 
is sometimes considerably 

more than the power actually used. Fig. 139 shows the 

wiring for this style of regulation. 






Fig- i39. 






97. The Leonard System of Motor Speed Control This 

system is especially advocated for use in operating elevators, 
cranes, battleship turrets, and all equipments requiring a 
thorough control of the speed and precision of stoppage. 
Fig. 140 shows the arrangement of such a system. M is 
a motor whose field is separately excited all the time from 
a source of constant potential, E. G is a dynamo which 
generates power for the armature of motor M. The arma- 
ture of the dynamo G is maintained at approximately con- 
stant speed by the prime mover S, which may be a steam 
engine or a motor run by power taken from the source E. 
The generator G is separately excited by current derived 
from E and controlled by the reversing rheostat C. 



MOTORS. 



177 



When it is desired to start the motor, the field of the 
generator is weakly excited by moving the controller so 
that a high resistance is in circuit with the field. This 
causes the dynamo to send current of low potential to the 
armature of the motor M. The latter then starts to move 
slowly. To accelerate the speed, more resistance is cut 
out of the controller. The pressure of the current supplied 
to the motor armature simultaneously increases and with 
it the motor's speed. Since the power represented by 
the current required to excite the field of G is at most 




Fig. 140. 

but a small fraction of the useful power given out by the 
motor, the I 2 R loss in the resistance C is very much less 
than would be the loss in a series regulating resistance as 
described in the last section. It is claimed that the extra 
first cost of this system is offset by the decreased cost of 
repairs, since violent stresses and bad sparking are avoided. 



178 DYNAMO ELECTRIC MACHINERY. 

98. Slow-Speed Motors It is a practical advantage to 

have a motor connected directly to the machine it is to 
operate, without the intervention of belting or reducing 
gears. Slow speed is also of advantage where absence of 
jarring is desired or where many stops and starts are to 
be made. Slow speed can always be obtained from an 
electric motor ; but it is generally expensive, since the 
natural speed of motors as well as of dynamos is high. In- 
crease of magnetic flux and increase of armature diameter 
is necessary to obtain slow speed. The increase of ma- 




Fig. 141. 



terial increases both the first cost and the losses during 
operation. 

The power of a motor is its torque or turning moment 
multiplied by the number of revolutions ; hence for the 



MOTORS. 



179 



same output of work, a machine making half as many 
revolutions as another must have twice the turning mo- 
ment. These conditions make it imperative that the ma- 
terials of construction, both iron and copper, be worked at 
maximum magnetic and current densities respectively, in 
order to economize in first cost and weight. In general 
the efficiencies of low- 
speed motors do not 
compare favorably 
with those of motors 
having a higher speed. 
Fig. 141 is a cut of a 
Crocker Wheeler eight- 
pole, direct current 
motor for direct connec- 
tion. It will furnish 2 
horse-power at 100 rev- 
olutions per minute, 4 
horse-power at 200 R. 
P.M., and the quotient 
of its speed per minute 
by its full load horse- 
power is equal to the 
constant quantity 50. Its efficiency increases as the 
speed according to the curves shown in Fig. 142. 



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100 200 300 100 500 600 
Fig. 142. 



99. Brake Motors. — For cranes, elevators, and hoists, 
where it is necessary to hold the load after raising it, and 
for looms and printing-presses, where it is important to 
secure a sudden and accurate stop instead of a gradual 
slowing down, it is desirable to use motors with a brake 
attachment. A brake operated by hand or foot requires 



i8o 



DYNAMO ELECTRIC MACHINERY. 



careful operation lest it be applied too soon and injure the 
machine, or too late and allow the load to fall some ; hence 
an automatic brake is desirable. 

Fig. 143 shows the construction used by the Crocker 
Wheeler Company. One of the pole pieces is pivoted at 
its base, and thus has a slight motion to or from the arma- 
ture. It is normally held from the armature by a heavy 
coil spring, and in this position tightens the brake band. 
The moment that current is allowed to pass through the 
field coils, the poles attract each other, overcoming the 



mmitimmu 




Fig. 143. 



resistance of the spring, and the brake band is thus loosened. 
The spring and band may be adjusted to allow a few revo- 
lutions before stopping, or the armature may be clamped 
the instant current is turned off. In the latter case, if 
connected to heavy machinery, shafting or gearing may be 
broken. 

Strap brakes are cumulative in their action ; the friction 
on the free end of the brake against the " drum tightens 
the whole brake, thus increasing its effect. This action is 



MOTORS. 



181 




~J 



OFF 
Fig. 144. 



only obtained when the motion of the drum is away from 

the fixed end. To obtain powerful brake action, therefore, 

on motors that run either way, 

as in elevator motors, a reversing 

brake is employed. This is op- 
erated by the movement of one 

pole piece as before, but the ends 

of the brake band are attached 

to a system of links and levers 

so that either end may become 

the fixed end. This construction 

is shown diagrammaticallyin Fig. 

1 44. When the brake is applied, 

the friction causes the whole 

band to follow the drum until the sliding link attached to 

one or the other end of the band is held by the stud. The 

other, or free, end of 
the band, is tightened 
by the pull of the lev- 
ers on one of the 
smaller straps attach- 
ed to the brake band 
as shown. Fig. 145 
shows a one horse- 
power Crocker Wheel- 
er motor fitted with 
reversing brake. 

In multipolar ma- 
chines it is imprac- 
ticable to employ a 
moving piece, and in 
Fig# I45# large bipolar ma- 




182 



DYNAMO ELECTRIC MACHINERY. 



chines it is undesirable to interrupt the magnetic circuit 
by a pivot joint; hence a solenoid brake is employed. This 
is simply a spring actuated friction brake kept norm- 
ally in engagement. On current being supplied to the 
machine a solenoid acts to release the brake. The opera- 
tion of this type is clearly seen by inspecting Fig. 146. 




Fig. 146. 

An objection to this type of automatic brake is that it 
consumes electrical energy all the time that the machine is 
in motion. 



100. Recording Meters. — The recording watt-hour meter, 
Fig. 147, is coming into extensive use, both as a station in- 
strument and as a measurer of the quantity of energy 
supplied to individual consumers. It is a very delicately 
adjusted compound-wound motor, having no iron in its 
magnetic circuit. When a current flows, the time integral 



MOTORS. 



183 



of the watts or power is registered, by means of a train of 
wheels operated by the armature, on a dial similar to that 
of a gas-meter. The connec- 
tions for such a meter are 
shown in Fig. 148. The ar- 
mature is connected in series 
with a high resistance across 
the service wires ; hence the 
current flowing in the arma- 
ture is proportional to the 
volts of the supply. The 
field coils are in series with 
the service, giving a field 
strength proportional to the 
current, and the motor effort 
is proportional to the product of the two or to the watts 
supplied. The shunt field coils are added to compensate 
for the friction of the moving parts. Since a small cur- 
rent is always flowing in the armature, as well as in the 




FROM GENERATOR 



Fig. i47. 




shunt field coils, the motor is always slightly excited, and 
by regulating the number of shunt turns the amount of 



1 84 DYNAMO ELECTRIC MACHINERY. 

this excitation is adjusted so that at no load on the service 
wires the armature almost, but not quite, moves. If it 
were not for this constant excitation, a small, though 
continuous, current could be drawn off the mains without 
operating the recording mechanism because of its friction. 
To control the speed a copper disk is mounted on the 
armature shaft and between the poles of two or more 
adjustable and permanent horseshoe magnets. When the 
armature revolves, Foucault currents are set up in this 
plate, and cause the proper retardation. By moving the 
poles of these horseshoe magnets from the center to the 
circumference of the disk, a variation of about 16 per cent 
in the speed for a given watt consumption can be effected. 
Advantage is taken of this fact in adjusting the instru- 
ments. The more important bearings are constructed of 
jewels, such as are used in watches, and the whole machine 
is carefully protected from dust. When the instrument 
is in a position where it is subject to jars or vibrations 
that reduce the friction of standing to such a point that 
the constant excitation causes the armature to revolve a 
little, the machine is said to "creep." The remedy is 
to mount on a rubber or other non -vibrating base, or to 
reduce the number of shunt field turns. 



SERIES MOTORS. 185 



CHAPTER X. 

SERIES MOTORS. 

1 01 . Series Motors. — When subjected to a heavy load on 
starting, that is when there is a heavy current at a very low « 
speed, a series-wound machine is far superior to one that is 
shunt-wound. For work that requires good effort at widely 
different speeds the series motor is particularly adapted. 
For this reason series-wound machines are used on electric 
railways, for crane motors, for ammunition and other hoists,. 
for mill motors, and in all other places where a good effort 
is required at a varying speeds. A series-wound machine 
can be used on either a constant current circuit or on a con- 
stant potential circuit ; but a series motor is seldom run on 

a constant potential circuit unless it is directly or very 
solidly coupled with its load, as in the case, for instance, 
of a railroad motor. If connected by means of a belt, 
and if the belt should break off or slip off, the motor would 
race and damage might result. This difficulty does not 
present itself when series motors are used on a constant 
current circuit. 

102. Series Motors on Constant Potential Circuits. — As 

in the case of a shunt motor, on a constant pressure 
circuit, the armature speed of a series motor will increase 
until it reaches a value where the counter E.M.F. cuts 
down the armature current to such a point that the total 



86 



DYNAMO ELECTRIC MACHINERY. 



electric power, (IE), received by the motor, is equal to the 
sum of the fixed losses, the variable losses, and the useful 
mechanical power. With a shunt-wound motor, a very 
small variation of speed is sufficient to compensate for a 
wide variation of load. A series motor tends to increase 
its speed on removal of the load, as in the case with shunt 
motors. It in this manner increases the counter E.M.F. 
The resulting decrease of current results, however, in a 

weakening of the field, 
and as a consequence ad- 
ditional speed is required 
to maintain the E.M.F. 
Thus a small change in 
load results in a wide 
change of speed in a series 
motor. For a series- 
wound mill motor, the 
relations which exist be- 
tween speed, current, and 
useful torque (turning mo- 
ment) are shown in Fig. 
149. There is also given 
a curve of the efficiency of the machine including gear- 
ing. It is evident that if, while the motor is at rest, 
the circuit be closed, an enormous rush of current would 
occur, giving an enormous torque. Destructive heating 
and sparking would probably result. To prevent damage 
it is therefore necessary, in the operation of these motors, 
to insert a series resistance at the start which may be cut 
out after the speed has risen enough to give a sufficient 
counter E.M.F. In practice controllers are used as de- 
scribed later. 



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SIZE 14 280 V. 

MILL MOTOR SERIES WOUND 

CROCKER-WHEELER COMPANY 
























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SERIES MOTORS. 187 

103. Railroad Motors Experience has shown that 

series motors operating on a constant potential circuit of 
550 volts, furnish a very satisfactory motive power for the 
propulsion of trolley street-cars and electric railroad motor- 
cars. At the time of this writing there are nearly two 
million horse-power of street-car motors in service in this 
country. The railway motor has been developed to a high 
degree of perfection during recent years, and is reasonably 
well fitted to meet the many requirements that are found 
in this service. A railway motor must be mechanically 
strong to withstand the excessive hammering to which it 
will be subjected when in service. Rough tracks and bad 
switches are usual in trolley road beds. When satisfactor- 
ily geared to the wheel axle, the motor can be suspended 
by springs on one side only, the other side being of neces- 
sity mounted directly on the axle. Railway motors are 
also subject to abuse at the hands of the motormen. The 
series resistance is often cut out rapidly before the car has 
an opportunity to accelerate. As a result there is an enor- 
mous current and torque with little speed. This severely 
strains the motor and is particularly liable to disturb the 
armature windings. The motor must be either weatherproof 
of itself or incased in a weatherproof shell, because of the 
mud, the water and the slush through which cars must 
often run. Furthermore a railway motor should permit of 
quick, convenient, and accurate alignment of parts and 
adjustment of the intermediate driving mechanism. 

The method of suspending the motors from the trucks 
is a matter of considerable importance. In practice four 
styles of suspension are used, viz., the side bar, the cradle, 
the nose, and the yoke suspensions. In every case one 
end of the motor frame contains bearings which run on 



DYNAMO ELECTRIC MACHINERY. 




SERIES MOTORS. 



189 




190 



DYNAMO ELECTRIC MACHINERY. 



the wheel axle and keep the pitch circle of the armature 
shaft pinion always tangent to the pitch circle of the gear 
which is mounted on the axle. The side-bar suspension, 
shown in Fig. 150, consists of two parallel side-bars which 
are mounted on the truck through heavy springs, and 
which support the motor in the line of its center of gravity. 
The motor-axle bearings are thus relieved of the weight of 
the motor, and the latter is held without undue strains. 




Fig. 153. 

The cradle suspension, Fig. 151, is very similar to the side 
bar, the difference being that the two side bars are replaced 
by one U-shaped piece. This, at its curved end, is mounted 
flexibly on a part of the truck frame which in turn is 
mounted on the truck through springs. The nose suspen- 
sion, Fig. 1 5 2, does not hold the machine at its center of 
gravity, but part of the weight is thrown on the motor- 
axle bearings. The rest is suspended from a spring- 
mounted member of the truck bv a link, bolted to a "nose" 



SERIES MOTORS. 



IQI 



cast in the motor frame. The yoke suspension, which is 
the least flexible of all, differs from the nose suspension in 
that the link is dispensed with and the spring-supported 
member of the truck is bolted rigidly to the motor frame. 
The cradle-suspension type is advocated by the Westing- 
house Company, the yoke or nose by the General Electric 
Company. The size or style of truck frequently requires a 
particular type of suspension. 




Fig. 154. 



A GE-67 railway motor, made by the General Electric 
Company, is shown in Figs. 153 and 154. This machine 
will develop 38 horse-power when operated on a 500-volt 
circuit without heating more than 75° C. above the sur- 
rounding atmosphere after one hour's run. The magnet 
frame is hexagonal, with rounded corners, and is cast in 
two pieces from soft steel of high permeability. The 
parts are hinged together so that the lower part may be 



192 



DYNAMO ELECTRIC MACHINERY. 



swung down for inspection or repairs (Fig. 155). The up- 
per part has cast on it two lugs, shown clearly in Fig. 153, 
pierced with two holes each for bolting to the yoke. Nose 
suspension can, however, be substituted. A covered opening 
over the commutator permits removal of the brushes with- 
out disturbing the rest of the machine. The bearings, 
both for armature shaft and for axle, consist of cast-iron 




Fig. 155. 

rings or shells, with Babbitt metal swaged into them, and 
are arranged for lubrication by both oil and grease. The 
oil is supplied to the shafts by felt wicks leading from oil- 
wells. The grease is fed through a slotted opening in the 
top of each bearing from a grease-box directly over each 
bearing, and means is provided for the passage of the 
lubricant from the bearing after it has been used. The 
armature bearings are 3" x 8" at the pinion end and 2% " x 






SERIES MOTORS. 193 

6\" at the commutator end. The motor-axle bearings are 
each 8" long. The pole pieces are built of soft iron lam- 
inations, riveted together, and are securely bolted into 
place on the magnet frame. The coils are spool wound, 
and are held in place by steel flanges. These magnetically 
imperfect constructions are rendered necessary by the 
severe service the machine is expected to stand. 

The armature is built up of thin, soft iron laminations, 
japanned, and keyed to the shaft. At each end is a cast- 
iron head, also keyed to the shaft. The core is hollow, 
ventilation being effected by air which enters at the pinion 
end and passes out through ventilating ducts left in the 
laminations. The winding is of the series drum type, 1 1 1 
coils being used, which are connected to a commutator of 
hi segments. The number of turns to a coil depends 
upon the class of service the motor is to render. The 
coils are made up of sets of three, each set being sepa- 
rately insulated before being placed in the slots. The 
coils are firmly secured in place by tinned steel wire bands 
held by chips and soldered together. Where the windings 
cross the ends of the core, they are protected by canvas. 
On the pinion end, a projecting flange protects the wind- 
ings from injury by careless handling. The brushes slide 
radially in finished ways in a brass brush holder, and 
are held in contact by independent pressure fingers. All 
the leads to the motor pass through rubber-bushed holes 
in the front of the magnet frame. The pinion has a taper 
fit on the armature shaft. It, as also the gear on the axle, 
is made of steel, and has teeth of 4-J" face and 3" pitch. 
When mounted properly on a truck with the ordinary 33- 
inch wheels, there is 4^" clearance between the bottom of 
the motor and the top of the rails. The shapes of the 



194 DYNAMO ELECTRIC MACHINERY. 

different parts of this motor are well shown in the exploded 
Fig. 156. 

A motor for railway service, very similar in design to 
the one just described, is No. 49, made by the Westing- 
house Electric and Manufacturing Company. This motor, 
shown in Fig. 157, has a weather-proof cast-steel frame, 
hinged to open in a horizontal plane through its center, 
and having a hand-hole above and one below the com- 
mutator. The upper half is cast with lugs for side-bar or 
cradle suspension, and also with a lug for nose suspension. 
The pole pieces are of laminated soft iron, are four in 
number, and are secured to the frame by having the latter 
cast around them. The lathe-wound field coils are secured 
on the pole pieces by brass castings, which are bolted to 
the frame. 

The armature is of the slotted drum type, having a 
laminated core with three ventilation passages parallel to 
the shaft. The coils are wound on formers, insulated in 
sets of two, and then applied to the core. This armature 
is constructed as light and as small in diameter as is prac- 
ticable, for the double reason of decreased centrifugal 
strain on the armature coils and decreased wear on the 
parts in stopping the car. When the motor is started, 
energy is stored in the armature and other revolving parts, 
as in a fly-wheel ; and when the car is stopped, this energy 
is wasted, and causes wear and tear on the pinions and 
bearings. In street-car service, where stops are frequent, 
this loss and this wear is by no means inconsiderable. 
Hence the armature of a street-railway motor should not 
be built with a great fly-wheel capacity. The high speed 
of car-motor armatures makes the operating expenses for 
car acceleration and retardation considerable. 






SERIES MOTORS 



195 




196 DYNAMO ELECTRIC MACHINERY. 




SERIES MOTORS. 



197 



104. Controllers. — It is general practice to equip each 
trolley car with at least two motors, and to regulate 
the speed of the car in the following manner : First, the 
two motors and a resistance are connected in series. The 
resistance is then cut out step by step until the two motors 
are operating in series on 500 volts. Since, with all the 



R 2 



MOTOR 1 . MOTOR 2. 

ARMA. FIELD ARMA, FIELD 



NOTCH i ' ••"--'-■ *m "»2 n 3 akma. field arma, field 

^-aw — m — aam, — ow — owf^* 
NOTCtt2 =w=M/w\, — m — om^-ow-* 
- j m — VW\rM/W\i — ow-owh-* 



RUNNING! 
NOTCH ) 



RUNNING) 
NOTCH f 



— A/VU — m — WH-Of — O-W-* 



-wa — vm — mr^-^yMr-- OtWt* 



-#i — m- 2 -^m- A <> ^ } <>Wi * 



RUNNING) 
NOTCH ) 



RUNNING) 
NOTCH j 



MAM WW — MAV J ^Vi h'<>^ll L T > 



-A/Wl A/VW mr J: -K>py 1 K>«Vr> 



Fig. 158. 

resistance cut out, there is no unnecessary I 2 R loss, this is 
called a running connection, and the controlling mechanism 
is said to be upon a running point. To further increase 
the speed, the motors are placed in parallel with a resist- 
ance in series with both. This resistance is then cut out 
step by step until the motors are each operating on 500 
volts. This, again, constitutes a running connection. A 
further change is sometimes effected by placing a small 



198 DYNAMO ELECTRIC MACHINERY. 

resistance in shunt with the fields when all the series re- 
sistance is out. This reduces the field flux, and causes a 
higher armature speed to maintain the necessary counter 
E.M.F. A car governed in this way has four running 
connections. On heavy cars, such as are used in elevated 
railway or inter-urban service, four motors are used on 
each car. In this case, the motors are governed in two 
series-parallel combinations, as if there were two separate 
cars governed by one controller. The connections for a 
two-motor car having nine speeds, a three-part series re- 
sistance, and a field-shunt resistance, are shown diagram- 
•matically in Fig. 158. 

The different connections are made by a motorman, who 
operates a handle on top of a controller. Each different 
combination is called a point or a notch. A pointer affixed 
to the controller handle indicates at what notch the car is 
running. Running points are indicated on the controller 
top by longer marks than the resistance points. A con- 
troller is almost invariably placed at each end of the car. 

Fig. 159 shows the interior of a General Electric 
Company's k-10 series parallel controller. The wires 
from the trolley, from the fields, from the armature, and 
from the different terminals of the series and shunt re- 
sistances are brought up under the car to terminals on a 
connecting-board in the bottom of the controller. On 
this connecting-board there are also switches, one for each 
motor. These enable one to cut out an injured motor 
without interfering with the operation of the other motor 
or motors. From the connecting-board conductors are 
run to terminals, called fi?igers or wipes. Mounted on an 
insulating cylinder, which may be revolved by the con- 
troller handle, are insulated contact pieces, which at various 



SERIES MOTORS. 



199 



angular positions of the cylinder make electrical connec- 
tions between various wipes, and give the proper con- 
nections for the various "points" or " notches." A 




Fig. 159. 

smaller cylinder connected to a reversing-lever, is situated 
to the right of the main cylinder. This has contact pieces 
which are arranged so as to enable the motorman to re- 
verse the direction of rotation of both motors or to cut 
them out entirely. Interlocking devices are supplied so 



200 DYNAMO ELECTRIC MACHINERY. 

that the reversing handle cannot be moved unless the con- 
trolling handle is in such a position that connection with 
the trolley is broken. The controlling handle also cannot 
be moved, if the reversing handle is not properly set either 
to go forward or to go backward. The reversing handle 
cannot be removed from the controller, save when the 
smaller cylinder is in the position that cuts out both motors. 
As serious arcs are liable to develop upon breaking a 
circuit of 500 volts, the contact pieces and wipes are sepa- 
rated from adjacent ones by strips of insulating material 
which are fastened to the inside of the controller cover, 
and which fold into place when the cover is closed. These 
are to be seen at the right of the figure. The power 
should never be turned off by a slow reverse movement of 
the controller handle, as destructive arcs are liable to 
occur upon a slow break. To lessen the speed of a car, 
the power should be completely and suddenly shut off. 
Before the car has slackened its speed too much the con- 
troller handle can be brought up to the proper point. The 
arcs, which form upon disconnection at the fingers, are 
pretty effectively blown out by the field of an electro- 
magnet whose coil is above the connecting-board at the 
right. 

105. Motors For Automobiles. — For electric automo- 
biles the series-wound motor is invariably employed. A 
storage battery of 40 or 44 cells is the customary source 
of power for these motors. The use of these cells affords 
a convenient and economical means of speed control. In 
the case of a single motor, for the first controller notch, 
the cells may be connected in four-series groups of 10 or 
11 each, giving about 22 volts, the four groups being con- 



SERIES MOTORS. 201 

nected in parallel. Other notches would correspond to 
other series parallel combinations, and finally the last and 
highest speed notch would correspond to a connection of 
all the cells in series. By this arrangement one cell is 
used just as much as any other, and they are discharged at 
equal rates. As the voltage supplied to the motor is 
varied without recourse to a series regulating resistance, 
there is no useless PR loss in starting or running at less 
than full speed. Often a series parallel control is employed 
when two motors are used. It is also common to use two 
37^ volt motors connected permanently in series and con- 
trolled as one motor. 

The advantage of using two motors on an automobile is 
that each may drive a wheel, allowing independent rota- 
tion on turning curves, while if one motor only is used 
some form of differential gear must be employed to allow 
for sharp turns. But the efficiency of one motor is in 
general greater than the efficiency of two motors of half 
the power, and the gain in efficiency by using one motor 
more than balances the cost and complication of a differ- 
ential gear. 

The question of efficiency in these motors is of great 
importance, for practice has shown that it is profitable to 
purchase I per cent efficiency, even at the cost of 10 per 
cent increase of motor weight. This is because the ratio 
of the battery weight to the motor weight is such that a 
decrease of i per cent in the capacity of the battery re- 
duces its weight more than 10 per cent of the motor 
weight. Since lightness is a prime object, only the very 
best materials can enter into the construction of a suc- 
cessful automobile motor. The magnetic circuit must be 
of material of the highest permeability. Ball bearings are 



202 DYNAMO ELECTRIC MACHINERY. 

not infrequently used in the shaft bearings, but their lia- 
bility to wear and the consequent regrinding is an objec- 
tion. 

It is general practice to rate these motors at 75 volts, or 
37J volts if two are used. Since 40 or 44 cells of battery in 
series can fall to 75 volts without injury, this is the lowest 
pressure on which the motors will be expected to run for 
any length of time at full speed. Hence this voltage is 
used as the basis for rating. For the best motors the 
rating is for a temperature rise of 50 or 6o° C. on an in- 
definite run. A motor so rated will carry 100 per cent 
overload for half an hour, 150 per cent for ten minutes, and 
a momentary overload of 400 or 500 without overheating 
or damage to the insulation. 

The battery of 40 or 44 cells is well adapted to automo- 
bile purposes. It can conveniently be made to have the 
required capacity, and it may be charged from any 115- 
volt direct current, incandescent lighting circuit with very 
little resistance in series and hence a small I 2 R loss. 

Although the voltage of these motors is somewhat low 
for the use of carbon brushes, the necessity of reversal of 
direction and the liability of sparking on over-load make 
their use desirable. Soft carbon brushes of low resistance 
can, however, be obtained, and they are to be recom- 
mended. 

Fig. 160 illustrates a motor which is used on automo- 
biles and manufactured by the Eddy Electric Manufacturing 
Company. It is a four-pole machine. The frame is ring 
shape and made of cast steel. The pole pieces, also made 
of cast steel, are fastened to the frame by bolts and 
steady pins. The armature is wound with formed coils 
which are cross connected, and therefore require but two 



SERIES MOTORS. 203 

sets of brushes. These brushes are made accessible by 
the existence of a window in the end plate. A pinion 
which is mounted upon the armature shaft meshes with 
an inside gear placed upon the wheel of the vehicle. A 
recess in the exterior of the magnet frame is fitted to re- 
ceive some part of the frame of the vehicle. Clamps for 
fastening to this frame are provided to suit the character of 
the vehicle. The motor is intended to be operated on 75 
volts, and is rated at 1.6 horse-power, at the speed of 1400 




Fig. 160. 

revolutions per minute. Its weight is 142 lbs., and it 
has an efficiency of 79^ per cent at full load. At 100 per 
cent over-load it has an efficiency of j6\ per cent, and at 
150 per cent over-load an efficiency of 73 per cent. 

106. Mill Motors For many kinds of mill work re- 
quiring great torque at low speed, reversibility, and wide 
variation of speed, the series-wound motor is well adapted. 
Since mill motors are to be used in places where dust, 
grit, and small particles of metal are apt to be floating in 
the air, it is necessary, to insure good continuous operation, 



204 



DYNAMO ELECTRIC MACHINERY. 





SERIES MOTORS. 



205 




iLl^—^ 




206 



DYNAMO ELECTRIC MACHINERY. 



that they be inclosed after the fashion of railway motors. 
Mill motors differ from shunt-wound machines in that they 
are capable of giving a turning-power, when slowed down 
or started from rest, many times as great as that given 
at full speed. 

Fig. 161 shows a Crocker Wheeler mill motor, and Fig. 
162 shows the same disassembled. It is a bipolar drum 
armature machine, designed for about 800 R.P.M., and 
giving without overheating an intermittent horse-power of 

1 4 or a continuous horse-power of 5 . 
It is rated in this way, since fre- 
quent stops and starts are expected 
in the use of such a motor. The 
hotter a motor gets during an in- 
terval of use the more it will cool 
off during an interval of rest. Of 
course an inclosed motor such as 
a mill motor heats up much more 
rapidly and severely than does an 
open motor where the air circulates 
around the fields and the armature. 
Since these motors are reversi- 
ble the brushes can have no lead. 
Sparkless running is accomplished 
by a long air gap. Being series 
wound the field increases with load 
and the speed is reduced corre- 
spondingly, hence commutation is readily effected. 

These motors are controlled by a variable series resist- 
ance, the various connections being made in a controller, 
such as is shown in Fig. 163. The circuits are made by con- 
tact pieces on a cylinder coming in contact with fingers or 





Fig. 163. 



SERIES MOTORS 



207 



wipers which are mounted on a board forming the back of 
the controller. The controller illustrated is also a reverser. 
The motor can be run in either direction by moving the 
controller handle to the right or to the left of the central 
position. 



208 DYNAMO ELECTRIC MACHINERY. 



CHAPTER XI. 

DYNAMOTORS, MOTOR-GENERATORS, 
AND BOOSTERS. 

107. Dynamotors. — A dynamotor is a transforming 
device combining both motor and generator action in one 
magnetic field, with two armatures or with an armature 
having two separate windings. They are generally sup- 
plied with a commutator at each end, which are connected 
to the two windings respectively. Either winding of the 
armature may be used as a motor winding, and the other 
as the dynamo winding. These machines occupy the same 
position as regards direct current practice as is occupied 
by transformers in alternating current practice. That is, 
they enable one to take electrical energy from a system 
of supply at one voltage, and deliver it at another voltage 
to a circuit where it is to be utilized. They cannot, how- 
ever, be constructed so as to operate with the same high 
efficiency as a transformer does. As the currents in the 
two armatures flow in opposite directions, and the machines 
are so designed as to have practically the same number of 
armature ampere turns when in operation, there is practi- 
cally no armature reaction. The field, therefore, is not 
distorted so as to require a shifting of the brushes, nor is 
there sparking present as a result of a change of load. 
These machines are more efficient than motor generators, 
which will be described later, as they have but a single 



DYNAMOTORS. 209 

field. They cannot be compounded so as to yield a con- 
stant E.M.F. at the dynamo end. A cumulative series 
coil would tend to raise the E.M.F. at the dynamo end, 
but it would lower the speed of the armature as a motor 
by a corresponding amount. 

108. The Bullock Teaser System. — Dynamotors are 
used extensively by the Bullock Electric Manufacturing 
Company in their so-called Teaser system of motor-speed 
control. This system is used in driving large printing- 
presses from supply circuits, which are at the same time 
used for lighting and other purposes. Large printing- 
presses contain very many sets of gears, and possess very 
large moments of inertia. These large machines require 
an unusually large torque on the part of the motor to start 
them. Sometimes it is as much as five or six times that 
torque which the motor is called upon to produce at full 
load. Now, the torque which is exerted by a motor is de- 
pendent upon the current which flows through its arma- 
ture, while the speed at which this torque is applied is 
dependent upon the impressed electromotive force. As 
the current, which is required to produce the normal run- 
ning torque is already of considerable strength, it is desira- 
ble that some direct current electrical transformation be 
employed to avoid the excessive starting current. The 
Teaser system accomplishes this by making use of the- 
dynamotor. The motor winding is designed for five times 
the electromotive force of the dynamo winding. Its field 
winding is excited directly from the supply mains. The 
negative brush of the motor side is connected with the 
positive brush of the dynamo side. The two armature 
windings are connected in series with a regulating resist- 



2IO 



DYNAMO ELECTRIC MACHINERY. 



ance to the supply mains. At starting, the main motor, 
which drives the press and which is generally a cumu- 
latively compound-wound motor, is supplied with current 
from the dynamo end of the dynamotor. The voltage 
with which it is supplied is somewhat less than one -fifth 
that of the main supply, depending upon the magnitude 
of the resistance in series with the dynamotor. This low 




voltage permits of the application of a proper amount of 
torque at a low speed. Furthermore, the drain of current 
from the supply mains is but about one-fifth that which 
passes through the main motor. By manipulating the dy- 
namo regulating resistance, the electromotive force sup- 
plied to the main motor is raised, and with it the speed. 
The highest speed of the main motor which can be attained 
by this arrangement is such, that, when attained, the mo- 
tor's connections may be transferred to the supply mains 



DYNAMOTORS. 



211 



FIELD COIL 

nwmw 



AMAAMAAAA 



TEAZER 
ARMATURE 




through another series regulating resistance without any 
excessive drain of current from those mains. The arrange- 
ment of the apparatus is shown in Fig. 165, and the 
amount of current which is 
taken by the main motor as 
compared with the amount 
of current which is drawn 
from the supply mains is re- 
presented in Fig. 164. Re- 
gulation of the resistances 
and changes of connection 
are accomplished through the 
aid of a controller. The 
different speeds are secured 
by the manipulation of a 
single hand-wheel on the con- 
troller, and thus the press- 
man has at his command a means of manipulating the press 
which is not complicated. 



'WWWWJ 



Vmain motor\ 
Xarmature A 



Fig. 165. 



109. Dynamotors for Electro-Deposition of Metals. — In 

large electro-plating establishments, it is common to in- 
troduce a dynamotor, whose two armature circuits are 
exactly similar, and under ordinary excitation give 5 or 10 
volts. The commutators, brushes, collecting devices, and 
leads are of necessity quite massive. The leads are gener- 
ally so arranged that the two armatures may be placed in 
series with each other, or in multiple. The low voltage 
of platers makes it impracticable to have a machine self- 
exciting. It is common practice in cities to excite these 
machines from no-volt lighting circuits, with a regulating 
rheostat whose resistance is of such a magnitude as to 



212 



DYNAMO ELECTRIC MACHINERY. 



permit of the variation of the voltage of the machine over 
a range of 25 per cent of its full-load value. Fig. 166 
shows a dynamotor constructed by the Eddy Electric 
Manufacturing Company for the electro-deposition of cop- 
per. Each armature winding gives 10 volts and 4,500 




Fig. 166. 

amperes. It is designed to be belt-anven through a large 
pulley at one end of the armature shaft. A small pulley 
upon the other end is for the purpose of receiving a belt 
connected with a small separate exciter. The large split 
clamps connected with the leads are for the reception of 
the terminals of the main conductors. 



no. The Eddy Company's Rotary Equalizer This is 

a dynamotor having a single field which is excited from a 
220-volt circuit, and a single armature core upon which is 
wound two distinct 110-volt armatures. One armature 



DYNAMOTORS. 



213 



has its commutator on one end of the shaft and the other 
at the other end. The machine is used in connection with 
a 220-volt generator, to enable one to use it for supplying 
energy to a three-wire, no-volt, incandescent lighting sys- 
tem. The principle of its action can be seen from an 
inspection of Fig. 167, When the system is unbalanced, 




Fig. 167. 

that side which^ hasjhe smaller load has the lesser drop, 
and therefore the higher difference of potential The 
armature winding of the dynamotor which is connected 
with that side acts as a motor, runs the armature, and 
causes the other armature winding to act as a generator in 
raising the pressure of the heavier loaded side. Obvi- 
ously the employment of this system can, in some cases, 
result in a considerable saving of copper. 



in. Other Applications of Dynamotors. — The Crocker 
Wheeler Company manufactures a special line of dyna- 
motors (Fig. 168) for use in telegraphic work. The motor 
is designed to be supplied with electrical energy from street 
service mains, or from the house-lighting mains in the 
case of isolated plants. The generator end furnishes cur- 
rents at a constant potential, which is different in the case 



2I 4 



DYNAMO ELECTRIC MACHINERY. 



of different machines. These machines are designed to 
take the place of batteries of a large number of gravity 
cells such as were used, in large quantities, a few years ago. 
The cost of operation of a dynamotor for this service is 
about one-fifth of what it is in the case of the gravity 
cells. The space which the machine occupies is but 




Fig. 168. 



^oVo that of the cells. They are to be preferred to bat- 
teries also on the ground of cleanliness. Their reliability, 
when supplied by electric energy from large city service 
mains is equal to that of the cells. The same cannot be 
said in the case of small towns. The telephone companies 
are also rapidly adopting the dynamotor for the purpose 
of charging storage cells. With some forms, the charging 
of the cells can go on continuously, they being at the same 



DYNAMOTORS. 215 

time used for telephone purposes. Dynamotors also fur- 
nish a convenient and satisfactory means of heating 
surgeons' electro-cauteries. Cautery knives take from 3 
to 8 amperes at 5 volts, while dome cauteries take from 
1 5 to 20 amperes at the same voltage. 

112. Motor-Generators A motor generator is a trans- 
forming device consisting of two machines, a motor and 
generator, mechanically connected together. They have 
the advantage over dynamotors in that the voltage of the 




290 

Fig. 169. 

dynamo armature can be made to assume almost any 
value within limits by means of a resistance placed in 
series with its field-winding and capable of variation. 
They can furthermore, besides being separately excited, 
be shunt wound or compound wound. They are used 
quite extensively in the Ward-Leonard system of motor 
speed control, which was described in paragraph 97. They 
are also used for charging storage batteries. In this case 



2l6 DYNAMO ELECTRIC MACHINERY. 

they are almost always shunt wound. They are also used 
in electro-plating establishments. In this case they are 
separately excited, as the voltage generally employed in 
such places is too small to give satisfactory self-excitation. 
For general laboratory work on tests which require large 
current at a low voltage or a small current at a high 
voltage, motor generators are of inestimable value. 

113. Boosters A booster is a machine inserted in series 

in a circuit to change its voltage, and may be driven either 
by an electric motor, or otherwise. In the former case it is a 
motor-booster. This machine is used very extensively on 
Edison three-wire incandescent lighting systems which 
supply current at a constant potential. Feeders which run 
to feeding-points at a great distance, if supplied by current 
from the same bus bars as shorter feeders, will have too 
small a difference of potential at the feeding-points to give 
satisfactory service. A booster with its field and armature 
windings in series inserted in series in the feeder will add 
E.M.F. to the feeder which in magnitude is proportional to 
the current flowing in the feeder, that is, as the current in- 
creases the field excitation will increase and with it the 
E.M.F. produced by the armature. The machine may, 
therefore, be so designed as to just compensate for any 
drop which is due to the resistance of the feeders and to 
the current flowing through them. As all the current of the 
feeder must pass through the booster armature, the collect- 
ing devices must be massive and must be designed to carry 
these heavy currents. The rating of a booster is of course 
determined by the voltage which it produces, and the total 
current which passes through it and the feeders. Boosters 
are also used in tke central stations of trolley companies to 



DYNAMOTORS. 2\J 

raise the voltage which is supplied to the feeders connected 
with distant sections of the line. They are also being in- 
troduced in office buildings in connection with electric 
elevator service. When the elevator motors are supplied 
from the same generators as the lights and fans in an office 
building they give to the generators what is called a lumpy 
load. The excessive currents demanded by the elevator 
motors on starting produce wide fluctuations of voltage in 
the mains. A booster inserted in these mains may be 
made to add E.M.F. to the mains on these occasions. 



218 DYNAMO ELECTRIC MACHINERY. 



CHAPTER XII. 

MANAGEMENT OF MACHINES. 

114. Connections for Combined Output of Dynamos. — 
In general a dynamo is much more efficient when operated 
at its full load than when operated at one-half or one-quarter 
load. It is usual to install in central stations, which, as a 
rule, have to supply different quantities of electrical energy 
at different times of the day, a number of smaller units 
rather than one unit large enough to supply the total 
energy. By this means any load can be handled by a 
machine or a number of machines all operating at about their 
maximum of efficiency It is well, therefore, to consider 
the methods of combining two or more machines on one 
load. The simplest and most usual method of connecting 
dynamos is that employed in incandescent light generating 
stations. Here a number of constant pressure machines 
are arranged as in Fig. 1 70, to act in parallel on one pair of 
bus bars. The figure shows shunt machines with hand 
regulators. The various external circuits are connected in 
parallel to the bus bars. This practice is frequently 
modified by separating those machines which supply the 
circuits that deliver at the more distant points from those 
that operate the shorter circuits. This is because the main- 
taining of a constant and uniform pressure at all distribut- 
ing points requires a higher pressure on the station ends of 
the longer mains than on the shorter. When a machine 



MANAGEMENT OF MACHINES. 



219 



is to be thrown into circuit on to bus bars already in opera- 
tion, it is first brought up to speed, the field magnetization 
is then adjusted till the machine gives the same pressure 
as exists between the bus bars, and the main switch is then 



Mr 



k -^wwu 



Tvmm 



Fig. 170. 

closed, which puts the machine in circuit. The proper 
pressure at which to throw in the new machine may be 
roughly determined by comparing the relative brightness 
of its pilot lamp with that of the lamps operating on the 
circuit. 



+ BUS 



f\*> 



r -£MW 



r 



A more exact way is to 
compare the readings of a 
volt-meter across the ter- 
minals of the machine with 
one across the bus bars. 
The most convenient way is 
to use a "cutting-in galvano- 
meter." Of these there are 
two forms, the zero galvanometer and the differential gal- 
vanometer. The zero galvanometer, shown with connec- 
tions in Fig. 171, has a single coil of high resistance. 
When the pressure of the machine is not exactly that of 
the bus bars a current will flow one way or the other, and 



Fig. 171. 



220 



DYNAMO ELECTRIC MACHINERY. 



- — BUS 
L +BUS 


f 1 


* 


r 

1 G 
J 








-h 





the needle will be correspondingly deflected. When there 
is no deflection, the machine may be thrown into circuit. 
This instrument is simple and cheap, but it requires that 
one terminal of the machine be permanently connected to 
a bus, which is not always desirable. The differential gal- 
vanometer, Fig. 172, has two high resistance coils, one in 
shunt across the bus bars, and one in shunt across the 
machine terminals. 

When equal pressures are 
impressed on each of the 
coils, they, by their differ- 
ential action, hold the needle 
in equilibrium, but when one 
coil is subject to more pres- 
sure than the other a deflec- 
tion occurs. This instrument 
is more costly and more 
complex than the last, but it has the advantage that a two- 
pole switch may be used to cut in or out the machine. 

When shunt machines are connected in parallel, it is 
expected that they will all be kept at the same pressure. 
If they are not, no serious damage is likely to occur, since 
the lower pressure machine merely fails to take its full 
share of the load. If the pressure of one machine falls so 
low that it is overpowered and run as a motor, still no 
damage will result, save perhaps the blowing of a fuse, 
since the direction of rotation for a shunt machine is the 
same whether it be run as a dynamo or as a motor. If it 
be desired to regulate a number of machines together by 
one regulator, it may be accomplished by bringing the 
positive ends of the field coils to one side of the regulator 
and connecting the other side to the negative bus. 



Fig. 172. 



MANAGEMENT OF MACHINES. 



221 



Shunt machines may be operated in series by connecting 
the positive brush of one machine to the negative brush of 
the next, and connecting the extreme outside brushes with 
the line wires. When this is done each machine can be 
regulated separately to generate any portion of the pressure. 
If it be desired to regulate all the machines thus connected 
uniformly and as a unit, the field coils of all the machines 
may be put in series with one regulating rheostat, and 
shunted across the extreme brushes of the set. In the 
Edison three-wire system two 115-volt direct-connected 
shunt machines are mounted on one engine shaft. The 
dynamos are connected in series as described above, the 
neutral wire being connected to the united brushes, as in 
Fig. 173. 

Series-wound dynamos may be operated in series with- 
out any difficulty, though it is not customary to do so. 
Series generators are used almost exclusively on constant 
current (arc light) circuits, and it is usual to have as many 
machines as there are external circuits, each machine being 
of capacity enough to operate that circuit alone. A new 

form of Brush generator supplies 
several series circuits from its 
terminals, and regulates for all 
of them. If it be attempted to 
operate series dynamos in paral- 
lel, the following difficulty occurs : 
If the machines start with a 
proper distribution of load among 
them and one does not generate 
just its full pressure, then this one does not continue to take 
its full share of the load ; and, since it is series wound, 
a decrease in load is followed by a decrease in pressure. 




222 DYNAMO ELECTRIC MACHINERY. 

The conditions become always more uneven until the 
machine is overpowered and it turns into a motor. Since 
the direction of rotation of a series-wound motor is oppo- 
site to its direction when run as a dynamo, serious results 
may occur. The only remedy for this trouble is to arrange 
the field coils so that the magnetization in any one machine 
will remain the same as in the other machines, even though 
its pressure falls below that of the others. To accomplish 
this the series fields must all be placed in parallel. This 
may be done by means of an equalizer, which is a wire of 
small resistance connected across one set of brushes, and 
by placing the fields in parallel, as shown in Fig. 174. Two 




Fig. 174. 

series dynamos may be run in parallel without an equalizer 
by resorting to mutual excitatio7i, that is, by letting the cur- 
rent of one armature excite the field of the other. In this 
case if the pressure of one machine falls and its load there- 
fore decreases, the magnetization of the other is reduced, 
compelling the first to maintain its share of the load. 
Series dynamos are never operated in parallel in practice, 
but this discussion is introduced because of its application 
to compound-wound dynamos. 

The use of compound generators for constant pressure 
circuits is very common. Since these have series coils 
they cannot be run in parallel without special arrange. 



MANAGEMENT OF MACHINES. 



223 



ments. It is usual to fit the series coils with an equalizer, 
as in Fig. 175. The desired end might be accomplished 

in the case of two ma- . 

chines by making the 
series coils mutually ex- 
citing. 



<•>■ 



m 

[Si 



ssossm. 



ffLIZER J 



m. 



EIEUD C.0.IL8 



<tft 

r 



wssmsi 



Fig. 175- 



115. Connections of 
Motors for Combined 
Output. — Any number 
of shunt motors may be placed in parallel across mains 
of a constant pressure, and their operation will be sat- 
isfactory whether each has a separate load or whether 
they be connected through proper ratios to one shaft. 
Shunt motors will operate in series on a constant pressure 
circuit when positively connected together ; but if con- 
nected to the same shaft by belts, and one belt slips or 
comes off, that motor will race, and rob its mates of their 
proper portion of the voltage. This arrangement is not 
common. 

Series motors will operate satisfactorily on constant 
pressure circuits : but when two or more such machines, 
that are arranged in parallel on a constant pressure circuit, 
are connected to one shaft an equalizing connection is 
sometimes used. Series motors in series on constant 
pressure mains will operate satisfactorily, dividing up the 
total voltage between them according to the load each is 
carrying. If it be desired to make them share a load 
equally they must be geared together so that each rotates 
at the speed corresponding to its share of the voltage. 
Series motors only are used on constant current circuits. 
Any number of these may be placed in series on such a 



224 DYNAMO ELECTRIC MACHINERY. 

circuit individually or connected to a common shaft. A 
series motor on a constant current circuit may be over- 
loaded until it stops without harm, since a constant current 
flows at any speed. 

Compound-wound motors are coming into quite general 
use, and they are invariably operated on constant pressure 
circuits, and each machine has its own load. 

In ordinary electric railroad practice, as has been stated, 
there are two series-wound motors to a car, operating 
either in series or parallel, according to the position of the 
controller handle, on a constant pressure of 500 volts. 
Each of these motors is geared to a separate pair of driv- 
ing-wheels. Since under ordinary conditions the rate of 
rotation of the two motors is the same, the E.M.F. sup- 
plied to each is the same when they are in series, and 
since the current is common they divide the work evenly. 
When in parallel the pressure on each is 500 volts, and 
since the rotations are the same the currents will be the 
same and the load will be divided evenly. It often occurs 
that the back platform of a car is so loaded that the front 
drivers slip when the power is applied at starting. This 
occurs when the motors are in series and the current 
is common to the two. But the higher rate of rotation of 
the front motor causes it to generate a greater counter 
E.M.F., thus lowering the pressure acting on the rear 
motor. Thus more electric energy is consumed in the 
front motor, and the surplus of work turns into heat from 
the friction between the slipping wheels and rails. When 
the car gains such a velocity that the front wheels bite the 
rails, the work is again evenly distributed between the two 
motors. It should be remembered that this occurs only 
when the motors are in series. 



MANAGEMENT OF MACHINES. 225 

116. Care and Operation of Machines In what follows 

on the operation of motors and dynamos, it is assumed 
that the machine is properly designed and of sufficient 
capacity for the work it is called upon to perform. For 
satisfactory operation, the machine must be connected with 
an appropriate circuit and one of the voltage or amperage 
for which the machine was designed. Further it is 
assumed that the mere mechanical details have been looked 
to, such as proper foundation, proper alignment with shaft- 
ing, and good lubrication. Only electrical trouble will be 
treated. 

If trouble be detected, a machine should be at once 
stopped to prevent further trouble. In central generating 
stations, one of the most positive rules is not to shut 
down while any possible means is left to keep running. 
In such plants there are always one or two units held in 
reserve, and one of these may be started and substituted 
for a machine developing a fault so that the latter may be 
shut down and its fault remedied. 

Sparking at the brushes is the most general trouble, and 
it has more causes than any other. The brushes must 
make good contact with the commutator, they must be 
true, and have good contact surface. The commutator 
must be clean. Any collection of carbonized oil is sure to 
cause sparking. A very thin layer of good oil, free from 
dust, is advantageous. On a bipolar machine the brushes 
must be diametrically opposite, on a four pole exactly 90 
apart, etc. This condition must be attained while the 
machine is at rest, either by actual measurement or by 
counting the commutator bars between each brush. If 
the brushes of one set are "staggered" they may cover 
too much armature circumference and cause sparking. 



226 DYNAMO ELECTRIC MACHINERY. 

The brushes must be set at the proper point. This is 
accomplished while the machine is in operation under its 
required load. The rocker arm, which carries all the 
brush holders, is moved carefully back and forth until the 
point of minimum sparking is found. Sometimes there is 
quite an arc of movement in which sparking is not 
observed. The brushes should then be set at the center 
of this arc, since heating occurs when the brushes are off 
the proper commutating point, even if sparks be not seen. 

Sparking may be due to fault in the commutator. A 
high-bar, a low-bar, or flat, projecting mica, rough or 
grooved surface, eccentricity, or any condition of surface 
which causes the brushes to vibrate and lose contact with 
the commutator will surely cause sparking. If sparking 
be allowed to go without correction, it will pit the commu- 
tator and aggravate these conditions. If the irregularity 
of surface be slight, it may be cut down by sandpaper 
(never emery) held in a block cut to fit the commutator. 
If the surface be very bad, it must be cut down by a 
machine. A small armature may be swung in a lathe ; but 
a large one must be left in its own bearings, and a tool 
held against the commutator by some special device. A 
perfectly true commutator may act eccentrically toward 
the brushes because of wear in the shaft bearings. New 
bearings will remedy this fault. 

If a coil of the armature be short-circuited, periodic 
sparking may result. The coil is liable to burn out if the 
machine is not immediately stopped. The short circuit 
may occur from breakdown of the insulation within the 
armature, in which case rewinding is necessary ; or it may 
be caused by metal chips or the like at or near the com- 
mutator, in which case the cause can be easily removed. 



MANAGEMENT OF MACHINES. 227 

When a coil is broken very violent sparking occurs, since 
half the armature current is broken every time the com- 
mutator bar connected to the broken coil passes from under 
a brush. Such a break may occur within the armature, 
requiring rewinding ; but it is more likely to occur where 
the coil end is attached to the commutator bar lug. If 
the break be at this place, the wire needs but to be screwed 
or soldered to the lug and the machine is repaired. 

If the field of a motor is too weak, sufficient counter 
E.M.F. is not generated, and excessive current flows and 
causes sparking. The weakening of the fields may occur 
from a short circuit in the field coils or two or more 
grounds between the field coils and the pole piece, or by a 
broken field circuit (shunt coils) which reduces the mag- 
netism to almost zero. In any case, unless the trouble is 
to be found external to the coils, rewinding is necessary. 

Heating of machines is another frequent source of 
trouble. The limit of temperature that may be allowed in 
the bearings depends on the flashing-point of the lubricant 
used, but a well designed and lubricated bearing ought 
always to run cooler than the commutator or armature. 
The limit of temperature that may be allowed in the arma- 
ture depends on the " baking "-point of insulation used, 
and also on the melting-point of the solder used if the 
coil ends are soldered to the commutator lugs. A good 
general rule is this : If you can hold your hand on any 
part of the machine for more than a few seconds, that 
part is not dangerously hot. Of course metal feels warmer 
than insulating cotton for the same temperature, and 
allowance should be made for this. If a burning smell or 
smoke comes from a machine, the safe temperature limit 
has been far exceeded, and the machine should be shut 



228 DYNAMO ELECTRIC MACHINERY. 

down at once. This indicates a serious trouble — a short 
circut or a hot bearing probably. 

If the trouble arises from the bearings the ordinary me- 
chanical precautions of cleaning, aligning, lubricating, etc., 
will generally cure it. Never use water to cool hot bearings. 
If water gets into the windings of either the field or the 
armature, short circuits will occur and ruin the machine. 
It must not be assumed that because one part of a 
machine is hot the trouble lies with that part. Heat is 
quickly conducted all over a machine ; and when heat is 
detected in one place the machine should be felt all over, 
the hottest part probably being the part at fault. The 
brushes of a machine should not be set too tight, for, be- 
sides reducing the efficiency greatly, they cause much 
heat from friction. The commutator should not be more 
than 5 C. hotter than the armature. 

Machines that operate on constant pressure circuits are 
liable to overheat because of too much current flowing 
through some parts of them. This may result from over- 
loads, in which case the remedy is obvious, or because of 
short circuits in the machines, in which case rewinding is 
generally necessary. In the case of constant potential 
generators a short circuit of the mains will produce a sud- 
den and severe overload, which can only be remedied by 
tracing out the lines and removing the short circuit. 

When a machine makes an unwarranted amount of noise 
it usually indicates the need of attention. Carbon brushes 
chatter and spark sometimes when the commutator is 
sticky, the action being something like a bow on a violin 
string. Cleaning the commutator will cure this. Hum- 
ming and vibration result when the revolving parts are 
not revolved about their center of gravity. This may be 






MANAGEMENT OF MACHINES. 229 

because of faulty construction or warping after completion. 
If the fault be with the pulley, it may be turned out or 
counterweighted. If the shaft be sprung it may be 
straightened or a new one used. If the armature core or 
windings be out of balance, there is not much help for it. 
Slower speed will reduce the noise from this cause. 

Noise may occur from the armature rubbing or striking 
against the pole faces. This is a serious matter, and if not 
immediately attended to results in the destruction of the 
armature. It is caused generally by wear in the shaft 
bearings, in which case new brasses will remedy the 
trouble. Sometimes it results from a sprung shaft, in 
which case the shaft must be either straightened or re- 
placed. A rattle produced by loose collars, bolts, nuts, or 
connections would indicate that these parts needed setting 
up or adjusting. 

If a motor revolves too slowly, it may be because of an 
overload of mechanical work, because of excessive friction 
in the machine, or because of the armature rubbing against 
the pole face. A variation in the pressure supplied to a 
motor causes a variation in speed. If the field magnetism 
be too weak the motor will revolve too fast when not 
loaded, and too slow when under full load, and will take 
excessive current. A weak field may be caused by a short 
circuit which cuts out some or all of the field turns, or by 
a broken field circuit. If the load be removed from a 
series motor on a constant current circuit it will race badly 
unless its field coils are shunted. Practically such a motor 
should not be used in any position where it may be sud- 
denly relieved of its load, as by the slipping of a belt. A 
shunt motor, whose fields are not excited, will run either 
forward or backward when a current is allowed to flow in 



230 DYNAMO ELECTRIC MACHINERY. 

the armature, according to the relative magnitudes and 
directions of the residual magnetism and the armature re- 
actions. Ordinarily, however, if a motor runs backward, it 
may be assumed that the connections are wrong. Usu- 
ally changing the connections to the brush holders, so that 
the brushes change their signs without changing any other 
connections, will make the motor change its direction of 
rotation. A series motor also may be made to change its 
direction by changing the direction of current flow in 
either field coils or armature, but not in both. 

On starting up, a self-exciting dynamo is supposed to 
build up its voltage to normal, having at first no excitation 
save that of residual magnetism. After standing some 
time, or in proximity to other dynamos, or after being 
hammered, the magnet frame may have lost all its residual 
magnetism. In this case the machine does not build up 
when revolved. By passing a current from another 
machine through the field coils the dynamo will generate 
as a separately excited one. Then the exciting current 
may be thrown off and the self-excitation thrown on, when 
the machine will build up satisfactorily. If the residual 
magnetism becomes changed in direction, or the separately 
exciting current be passed in the wrong direction, then 
what little voltage may be generated will, when connected 
for self-excitation, send the current in such a direction as 
to tend to demagnetize the field, and building up will be 
impossible. A shunt machine builds up better the less 
the outside load, since at no load the terminal voltage is the 
greatest and the most likely to send a magnetizing current 
in the field coils. A series machine builds up better when 
the outside load is increased. Such a machine may even 
be momentarily short circuited to make it build up. For a 



MANAGEMENT OF MACHINES. 231 

given voltage resulting from residual magnetism, the current 
in the field coils is greater the less the resistance in the 
circuit. If the connections to one of the field coils in a 
bipolar machine be reversed, causing two poles of the same 
polarity, the machine will of course fail to generate. This 
condition may be detected by the use of a compass needle. 
Small machines sometimes generate at starting a few volts, 
showing proper connections and the presence of some 
residual magnetism, but refuse to build up beyond this 
point. It is sometimes convenient to materially increase 
the speed of such a machine, whereupon it will build up 
rapidly, and the speed may then be reduced to normal, and 
the dynamo will continue to generate at its normal pres- 
sure. 



232 DYNAMO ELECTRIC MACHINERY. 



CHAPTER XIII. 

THE DESIGN OF MACHINES. 

117. Different Methods of Design. — It is impossible to 
lay down a fixed set of rules to be followed in the design 
of dynamo electrical machinery. This is because the 
specified conditions of operation and construction are 
seldom alike in two cases. A designing engineer may be 
called upon to design a machine of a given output at a given 
voltage, the field frame, however, to be chosen from one of a 
set already in stock ; and again it may be required that the 
machine shall be direct connected, the output, the voltage, 
and the speed of rotation being given ; still, again, the 
capacity, the maximum gross weight, and the efficiencies of 
operation at various loads, may be specified, as in the case 
of an automobile motor ; or he may be called upon to design 
a machine of a given output and voltage, which shall 
operate at a satisfactory efficiency, and which shall have a 
first cost which will enable the manufacturer to successfully 
compete with others in the sale of his products. Through- 
out the calculations the engineer is obliged to refer to his 
experience or the experience of others in determining the 
values of different quantities which must be assumed before 
there can be any further progress on the design. Further- 
more, after having assumed certain values, results which 
are arrived at later on in the work will necessitate the re- 
jection of these values and the assumption of new ones. 



THE DESIGN OF MACHINES. 233 

Oftentimes what one might desire as a value for one quan- 
tity is undesirable, because it conflicts with the adoption of 
a value for some other quantity which is more desirable. In 
the following paragraphs a method is given for designing 
a machine under the conditions specified. 

118. Specifications. — The following specifications are 
given and must be complied with : 

The type of machine as regards the shape of its field 
frame, its bearings, and the method of its being driven ; its 
output in kilowatts ; its terminal voltage at full load and at 
no load ; the materials from which are to be constructed its 
field frame, its pole pieces, its armature core, its brushes, 
its shaft, its bearings, its armature spider, and its con- 
ductors ; and the insulation throughout its various parts. 

119. Preliminary Assumptions. — The design will be 
based upon an assumption of the values of four different 
quantities. 

The first assumption is that of the value of the flux den- 
sity in the air gap, which will be represented by (B^. The 
value which will be chosen will depend somewhat upon the 
method to be employed for obviating armature reaction. 
Almost all designers rely upon a stiff y bristly field \o> assist 
in preventing a distortion of the field when under load, and 
therefore higher flux densities are being used now than 
were a few years ago. Higher densities are used when the 
pole pieces are made of wrought iron or of cast steel than 
when they are made of cast iron. The densities are greater 
in the case of multipolar machines than in the case of 
bipolar ; and they increase, within limits, with the size of 
the machine. A value between 4000 and 7500 should be 
chosen. 



234 DYNAMO ELECTRIC MACHINERY. 

The second assumption is a value for the peripheral 
velocity V ) of the armature in feet per minute. The com- 
mon assumption in the case of drum armatures for all sizes 
above five k.w. is 3000 feet per minute. High-speed ring 
armatures have a higher value, ranging between 4000 and 
6000. The larger value is to be used in the case of large 
machines. 

The third assumption is a value for the current density 
in the armature conductor at full load. Inspection of a 
large number of machines shows the use in many of them 
of from 500 to 800 circular mils per ampere. Sometimes 
as small a cross-section as 200, and in other cases as large 
as 1200 circular mils per ampere, have been found. The 
low value is used in the case of machines subjected to 
periodic loads of short duration. This is the case with 
elevator motors, pump motors, sewing-machine motors, 
dental drill motors, and motors on special machinery. The 
high value is used on machines to be used in central stations 
for lighting or power purposes. The specified output in 
watts when divided by the full-load terminal volts gives the 
total current output of the machine at full load. This, 
divided by the number of armature circuits, gives the cur- 
rent which must be carried by each conductor at full load. 
This current multiplied by the assumed value of the number 
of circular mils per ampere gives the cross-section of the 
conductor in circular mils. Oftentimes a single armature 
conductor is made up of several wires in multiple. The 
multiplicity of wires affords pliability in winding, and 
obviates, to a certain extent, eddy currents. Again, the use 
of copper bars for windings is common, they being in- 
sulated by the use of micanite, fuller board, or other 
sheet insulating materials. A cross-section sketch of a 



THE DESIGN OF MACHINES. 235 

single conductor should be made in which the dimensions 
are given of the copper and of the insulating material. 

The fourth assumption is the value s to be given to the 
polar span, s represents the percentage of the armature 
circumference which is covered by the faces of the poles. 
This value varies considerably within narrow limits, but 
unless there is some special reason for the assumption of 
another value 0.8 may be taken. 

120. Design of the Armature 1. To determine the 

specific induced E.M.F. in volts per foot of active con- 
ductor. 

_, V 7.0.C M 1 

£ r = — -?— - X s& X 30.5 — s volts. 

60 9O0 IQ 8 

where the first term in the right-hand member represents 
the velocity of the moving conductor in centimeters per 
second, the second term represents the average induction 
density of the flux which enters the armature, and the 
third term consists of constants to reduce feet to centi- 
meters, and c. g. s. units to volts. 

II. To obtain the length of active conductor I' in feet. 

/'= -=7 X number of armature circuits. 
E 

III. To obtain the number of active conductors S upon 
the armature. 

Let ly = the number of layers in the armature winding. 

p = the assumed ratio of the length to the diameter of the 
armature core. 

d — the mean winding diameter of the armature in inches. 

w = the specific peripheral width of one armature conductor 
in inches. (In the case of a smooth-core armature 
w represents the width of the armature conductor 



236 DYNAMO ELECTRIC MACHINERY. 

plus the double thickness of its insulation, both in 
inches, while in the case of tooth armatures w rep- 
resents the width of one. tooth plus the width of 
one slot divided by the number of conductors in 
one slot in one layer.) 

The length of the armature core = pd inches = — ft. 

12 
and the circumference of the core = ird inches. The 

number of armature conductors in one layer = — hence 

w 



the total number of armature conductors S'=- 

Since 

.. . trd pd 
w 12 



dly 



Jl'w 1 2 
V Tv/yp ' 



_ izlyd _ irly Jl'w 1 2 _ J^ly 2 l'wi2 

V irlvp \ 



w w t ir/yp t urly-np 



-i 



1 2 irlyl' 
Wp 



In practice the width of the tooth ranges from 50 per 
cent to 80 per cent, the width of the slot. In some cases 
it has a width equal to that of the slot. The value for 5 
yielded by this formula must, in nearly all cases, be altered 
by either the addition or subtraction of a few conductors 
in order to make it possible to employ the type of winding 
which it seems desirable to adopt. The change may neces- 
sitate a slight alteration of one of the assumed values, and 
as a result the values derived from it. 

For machines whose speed is prescribed, as is the case 
with direct connected machines, one may use the form of 



THE DESIGN OF MACHINES. 237 

the formula 5 = — — , where d is to be obtained as de- 
w 

scribed in the end of the next paragraph. 

IV. To obtain the diameter of the armattcre d in inches. 

Sw 

d— -=■— inches. 

In case the speed of the armature in revolutions per 

minute V be prescribed, as is the case with direct connected 

machines, the preliminary assumption of the peripheral 

velocity V immediately gives a value for the armature 

diameter. yl ^ 

d — -— ft. = — — — inches. 
Vtt Ktt 

V. To determine the length of the armature I in inches. 

I = dp. 

VI. To determine the internal diameter of the armature 
core d' in inches. In determining this quantity a value for 
the flux density in the armature core ($> a must be assumed. 
Wiener states that in incandescent dynamos, in railway 
generators, in machines for power transmission and distri- 
bution, and in stationary and railway motors, the density 
varies from 5,500 to 15,500. Ring armatures have higher 
densities than drum armatures, low-speed machines higher 
densities than high-speed machines, and bipolar machines 
have larger densities than multipolars. & a = 8000 is a 
good assumption. 

If the machine have / pairs of poles, the flux which 
enters the armature through one pole 

_ / 7 r./CB / (2. 54 ) 2 



238 DYNAMO ELECTRIC MACHINERY. 

that is, the surface of the armature in square centimeters 
times the average gap density divided by the number of 
poles. Considering that but 75 per cent to 80 per cent of 
the length of the armature core is made up of iron, the 
rest being due to the spaces between the laminations and 
the width of the ventilating ducts, the radial depth of the 
armature core is 

d-d'= +° 



a '0.75 (2.54/ 



d'=d~ +< 



^0.75 (2.54)' 

- VII. To determine the armature losses. The armature 
as already determined would theoretically operate satis- 
factorily, but there is a possibility of its heating excessively 
when running under full load. There are the two constant 
supplies of heat, namely, that due to ohmic resistance 
and that due to hysteresis and eddy currents. There are 
also two avenues for the escape of heat, namely, radia- 
tion and air convection. An equilibrium is established when 
that temperature is reached which will make the escaping 
heat per unit of time equal to the amount of heat gen- 
erated in the same time. Concerning the escape of heat 
by radiation, it should be borne in mind that the watts 
radiated vary as the difference in temperature between the 
radiating body and the surrounding atmosphere and as the 
emissivity and the area of the radiating surface. There 
is also on starting a conduction of heat to neighboring 
bodies. After a short time, however, a static temperature 
condition will be established. The power loss in hysteresis 
in the armature is 

1 V 

P h = — v f> (V 6 — v watts. 
io 7 60 



THE DESIGN OF MACHINES. 239 

where rj equals the hysteretic constant of the iron (0.002), v 
equals the volume of the armature core in cubic centimeters. 
The assumption is made that the flux density in the arma- 
ture core is uniform. This is not true for the main core, as 
was shown by Goldsborough, and in the teeth the density is 
much greater. When the volume of the latter is a relatively 
large amount of the total core volume, a correction should 
be made. When making many designs, in which the same 
quality of iron is to be used, it is much easier to get the 
hysteresis loss per cubic inch at various densities from 
tables made up to suit the iron. The power loss due to 
ohmic resistance 



Pr = 



J max \ 2 r, 

rmature circuits/ ° 



number of armature circuits 



where I max is the full-load current of the machine in am- 
peres, and R a is the resistance of all the armature con- 
ductors arranged in series. Before getting P h and P r one 
must determine the values in VIII. to XL 

VIII. To obtain the armature speed V in revolutions per 
minute. This quantity is prescribed in the case of direct 
connected machines. In other cases in may be determined 
by the formula 



IX. To obtain the volume of the armature core v in cubic 
centimeters. 



v = zIttU—^ — j (2. 54 ) 3 , 



where z is a coefficient which represents that part of the 
armature core length which is occupied by iron. In ordi- 
nary laminations the space occupied by air and insulating 



240 DYNAMO ELECTRIC MACHINERY. 

oxide on the plates amounts to 10 per cent, therefore under 
these circumstances z = 0.9. The introduction of ventilat- 
ing ducts reduces this value by an amount which can be 
readily determined. 

X. To obtain the resistance of the armature wire in 
ohms. The total length of the armature wire, 

I t = /' x k, 

where k is a constant greater than unity, which takes into 
account the amount of dead wire employed in making the 
end connections. This value depends upon the value of 
p and upon the method of winding. In the case of formed 
coils its value may be determined from measurements upon 
a single coil. This value is generally slightly greater 
than 2. Considering that the resistance of a hot mil foot is 
1 1 . 5 ohms, the resistance of the armature 

1 1.5 l'k 



R„ = 



cross-section in circular mils. 



XI. To obtain the area of the armature radiating surface 
A in square indies, 



A=irld + 



XII. As 2 to 2\ watts can be radiated per square inch 
of armature surface without excessive heating, the value of 

P -\- P 

— r determines whether the armature is properly de- 
signed or not. If the fraction is less than 2, the armature 
is needlessly large, and should be redesigned. If the frac- 
tion is greater than 2\, the armature will heat excessively, 
and should also be redesigned. 



THE DESIGN OF MACHINES. 241 

121. Design of the Field. — XIII. Dimensions of the 
poles and field frame. The design of a field requires judg- 
ment and experience on the part of the designing engineer, 
and an acquaintance with the various machines of the type 
being designed. One must assume values for the following 
quantities : the flux density in the poles (& p , the flux 
density in the magnet frame ($> /f the coefficient of magnetic 
leakage X, and the ratio of the length of a pole to its diam- 
eter in case it has a circular cross-section, or to some other 
dimension in case it is not circular. The assumption is 
made here that the field frame is of a circular type, and 
that the pole is of circular cross-section. It is customary 
to choose such a value for (& p that the magnetization will 
be carried over the knee of the magnetization curve. In 
the case of %, however, it is customary to choose a value 
somewhat below the knee. The coefficient of magnetic 
leakage for this type of machine is 1.4. A careful design 
really requires a knowledge of the distribution of the leak- 
age flux. Long experience enables one to make allow- 
ance for this. From these assumed values one gets a 
value for the cross-section of a pole, 

A p = — — sq. centimeters, 

whence it follows that the diameter of the pole in inches 

dp = v inches, and the cross-section of the frame, 

2.54 V 7T 

A f = — — sq. centimeters. 

2 Qfrf 

XIV. Reluctance of the magnetic circuit. After mak- 
ing a provisional scale-drawing of the field-magnet frame 
with its poles and the armature core, exercising judgment 
derived from experience or from the inspection of other 



242 



DYNAMO ELECTRIC MACHINERY. 



drawings, determine the average length in centimeters of 
the path of the magnetic lines in the frame, in the poles, 
in the air gap, in the teeth, and in the armature core. 







Fig. 176. 

Represent by l f , l py l g , l e and l t the length in centimeters 
of the parts marked in Fig. 176. From the assumed 
values of the flux density, and from the magnetization 
curves of the metals from which the various parts of the 
magnetic circuit are constructed, one can get the respec- 
tive permeabilities. The reluctance may then be calculated 
as follows : 



Reluctance of the pole (R p = 



fi p A p 
of $ section of field frame (R f = 

7 
of the air gap Gi g = 



Pf A f 



of \ section of the armature core, 



<R„ = 



^zl{d-d'){2.^J 



THE DESIGN OF MACHINES. 



243 



To determine the reluctance offered by the teeth and 
winding-slots, it is convenient to assume that the total flux 
is carried by the teeth alone. Owing to the fringing of 
the field at the pole tips, not merely the teeth immediately 
under the pole face carry the flux from that pole, but, with 
very short air gaps, an extra tooth takes part in the trans- 



TABLE OF TOOTH-DENSITY CORRECTIONS. 



Corrected Iron 
Density. 


Densities on the 


Assumption that the Iron Transmits 
the Entire Flux. 


Lines per Sq. Cen- 
timeter. 


Tooth Width = 
Slot Width. 


Tooth Width = 
f Slot Width. 


Tooth Width = 
J Slot Width. 


17050 
18000 
19050 
20000 
21020 
22000 
23100 


17200 
18450 
19680 
21050 
22200 
24000 
26000 


17380 
18600 
20000 
21300 
23000 
24800 
26800 


I75IO 

18800 
20200 
21850 
2370O 

25500 
284OO 



21 

23 


CO 

E 22 

CO 

tu 

5 

> 20 

H 

CO 

z 

* 19 


UJ 

H 
O 

a 18 
tr 
O 









TOOTH 


DENSITY CORRECTION CURVES 
















T = WI 
S =WI 


OTH OF 
DTH OF 


TOOTH 
SLOT 






























-h* 
































^ 


^ 


y^ 
















V 


'/ 
















/6 


/ 


















V 


r 





















19 20 21 22 23 24, 25 

UNCORRECTED DENSITY' KILOLINES PER SQ, CM. 



Fig. 177. 



244 



DYNAMO ELECTRIC MACHINERY. 



mission. With large air gaps two or three extra teeth may- 
take part. The value of the permeability obtained from 
the flux density which is calculated upon the above as- 
sumption would be too small. The value of the reluc- 
tance based upon it would in consequence be too large. 
The flux density arrived at will have to be corrected by 
reference to the table on page 243. 

The permeability, fi t) corresponding to the corrected dens- 



20000 





s 


-\ ■** 


A ^U S 


V -^ 


r j? 


\ s 


X y 


n ^ 


t i^K 


A -&' 


L_ <£. 


X 7 


^ y 


X <r 


4-V- V T 


5 J? 


\? 


?\ 


-/ V k 


7 s 


7 ^ s 


-t v3^ 


1 - -4- $fe - 


' "| s. 


~1 N v 


t^ ^ 


- t v ^ 4- 


r ^ 


L ^ 



100 200 
10 20 



400 500 COO 700 800 900 1000 1100 1200 1300 1100 1500 IGOOjt' 
40 50 60 70 80 90 100 110 120 130 140 150 "160 // 

Fig. 178. 



ityand to be obtained from Fig. 178, should be inserted in 
the formula for the reluctance of the teeth, 

h 



<R,= 



PtA 



THE DESIGN OF MACHINES. 245 

where A t is the net iron cross-section of the teeth under 
one pole corrected for fringing. The reluctance, the flux 
through which must be maintained by the field- windings 
on one pole, is made up of a bi-parallel path in the arma- 
ture and a bi-parallel path in the field frame, both arranged 
in series with the pole, the gap, and the tooth reluctances. 
This reluctance is equal to 

3r+ «, + «, + «, + *=. 

2 p J 2 

XV. Magneto-motive force. The ampere turns per 
pole nl sh necessary to produce the flux <j> a in the armature 
at no load is equal to 

These ampere turns are furnished by the shunt coil on 
one pole. 

XVI. Shunt Coils. Assuming that E h volts are con- 
sumed in the field regulating rheostat, 



',% + «, + ^ + XCR* + ^] 



I sh 2p circular mils 
Whence, 

The cross-section in circular mils = ^- — " • 

E -E h 

Where n = number of turns in shunt coil, 

Ish = the current in the shunt at no load, and 
4j = the mean length of one field turn in feet. 

Assuming 1,000 circular mils per ampere in the shunt coil, 

_ circular mils 
A»= 1 and 

1000 



1000 nL 



ah 



circular mils 



246 DYNAMO ELECTRIC MACHINERY. 

From a wire table the space occupied by the n turns can 
be attained ; and, with due allowance for insulation, refer- 
ence to the preliminary drawing will enable one to deter- 
mine whether the assumed length of the pole l p is too 
small or too great. Space must be left for the compound 
coil. This occupies about one-half as much space as the 
shunt coil. If l p seems of unsuitable length it should be 
altered, and the calculation should be again gone over. 

XVII. Compound Coils. The method of calculating 
the number of compounding turns is so similar to that in 
the case of shunt coils that it need not be gone into in 
detail. The compound coils have to compensate at full 
load for drop in the armature, for drop in the series coil, 
for drop in the line in case of overcompounding, for the 
demagnetizing armature ampere turns, and for changes in 
reluctance due to skew by saturation. The back armature 
ampere turns, when multiplied by the coefficient of mag- 
netic leakage, give the series ampere turns necessary to 
compensate for them. It should be borne in mind that 
the maximum possible lead brings the brush no farther 
than the pole tip. To compensate for a drop of a certain 
percentage requires that the density in the air gap be 
raised by that same percentage. This necessitates an 
increase of all the densities. The increase of each reluc- 
tance and the increase of each corresponding flux must be 
cared for by the series windings. The coefficient of mag- 
netic leakage varies with the load. The manner of its 
variation may be unknown. The reluctances, into which 
it enters, are such a small per cent of the total, that its 
variation may often be neglected. 

The following blank form, to be filled in by students in 
designing, is self-explanatory. 



THE DESIGN OF MACHINES. 



247 



POLYTECHNIC INSTITUTE OF BROOKLYN. 

DEPARTMENT OF ELECTRICAL ENGINEERING. 

Data sheet to be filled in by students taking Electrical Engineering 8, and 
covering the work of the first semester. This imist be accompanied by the 
following scale drawings : end elevation, longitudinal cross-section, plan, and 
important details. A diagram of the armature-winding must also be given. 
Assumed values are to be entered in red ink. 





Designer 




Submitted , 




SPECIFICATIONS. 


25. 


Internal diameter of core 


!_ 


Type of Machine 


26. 


Length over conductors 


2. 


Number of poles 


27. 


Length of core over laminations 


3- 


Capacity in kilowatts 




and ducts 


4- 


Terminal volts at no load 


28. 


Net length of iron 


5- 


Terminal volts at full load 


29. 


Number of ventilating-ducts 


6. 


Amperes at full load 


30. 


Width of each ventilating-duct 


7- 


Revolutions per minute 


3*« 


Thickness of sheets 






3 2 - 


Number of slots 




MATERIALS. 


33- 


Depth of slots 


8. 


Armature core 


34. 


Width of slot at root 


9- 


Armature spider 


35- 


Width of slot at. surface 


10. 


Armature end plates 


36- 


Width of tooth at root 


11. 


Armature shaft 


37- 


Width of tooth at armature 


12. 


Commutator segments 




face 


ij- 




38. 


Size and shape of bare con- 


14- 


Magnet frame 




ductor 


15- 
16. 


Pole piece 
Pole shoe 


39- 


Size of conductor insulated 


40. 


Pitch of winding, No. of teeth 


17- 


Brushes 


41. 


Arrangement of wires or bars 


iS. 


Brush-holders 




per slot 


19. 


Brush-holder yoke 


42. 


Number in parallel per slot 


20. 


Commutator insulation 


43- 


Number in series per slot 


21. 


Armature conductor insulation 


44- 


Total insulation between con- 


22. 


Field-coil insulation 




ductors 






45- 


Thickness insulation between 




DIMENSIONS. 




conductors 




Armature. 






23- 


Diameter over all 




Air Gap. 


24. 


Diameter at bottom of slots 


46. 


Length in center 



248 



DYNAMO ELECTRIC MACHINERY. 



47. Length maximum 

48. Bore of field 

49. Minimum clearance 

Pole Shoe. 

50. Length parallel to shaft 

51. Length of maximum arc 

52. Length of minimum arc 

53- 



57- 
58. 

59- 
60. 

61. 
62. 

63- 
64. 
65. 
66. 



67- 
68. 
69. 

70. 

7i- 

72. 

73- 



Minimum thickness 



Poles. 

54. Length of pole 

55. Width or diameter of pole 

56. Lenpth parallel to shaft 

Magnet Spool. 
Number of spools 
Length over all 
Length of winding-space 
Depth of winding-space 



Magnet Frame. 
External diameter 
Internal diameter 
Thickness 
Diameter over ribs 
Thickness of ribs 
Length along armature 



Commutator. 
Diameter 

Number of segments 
Width of segment at commu 

tat or face 
Width of segment at root 
Useful depth of segment 
Thickness of mica insulation 
Available length of surface of 

segments 

74. Total length of commutator 

75. Peripheral speed 

Brushes. 

76. Number of sets of brushes 

77. Number in one set 



78. Length 

79. Width 

80. Thickness 

81. Area of contact one brush 

ELECTRICAL. 
Arn ature. 

82. Voltage at no load 

83. Total voltage at full load 

84. Total current 

85. Number of sections 

86. Turns per section 

87. Number of layers 

88. Total number of inductors 

89. Type of winding . . . circuits 

90. Style of winding 

91. Circular mils per ampere 

92. Mean length of one turn 

93. Total length of arm wire 

94. Resistance of armature cold at 

20 C. . . . ohms 

95. Resistance of armature hot at 

70 C. . . . ohms 

Shunt Coils. 

96. Size of wire, No. B. & S. Gauge 

97. Turns per layer 

98. No. of layers 

99. Turns per spool 

100. Mean length of one turn 

101. Total turns 

102. Total length of wire 

103. Total weight of wire 

104. Total resistance at . . . 20 C. 

ohms 

105. Total resistance at . . . 70 C. 

. . . ohms. 

106. Volts allowed for rheostat 

107. Maximum current . . . am- 

peres 

108. Total ampere turns 

109. Circular mils per ampere 









THE DESIGN OF MACHINES. 



249 





Series Coils. 


1 33- 


no. 


Size and shape of conductor 




III. 


Number of conductors in mul 
tiple 


134- 


112. 


Arrangement 




"3- 


Turns per layer 




114. 


Number of layers 


135. 


"5- 


Turns per spool 




116. 


Mean length of one turn 


136. 


117. 


Total turns 




118. 


Total length of conductor 


137. 


119. 


Total resistance at . . . 20 C. 






. . . ohms. 


I38. 


120. 


Total resistance at . . . 70 C. 






. . , ohms. 


139 


121. 


Maximum current . . . am- 


140 




peres 


141 


122. 


Total ampere turns 




* 2 3 


Circular mils per ampere 

HEATING, 


142 



Armature- 

124. Area of drum radiating sur- 

face . . . sq. in. 

125. Area each end radiating sur- 

face . . . sq. in. 

126. Total radiating surface . . . 

sq. in. 

127. I 2 Riu\\ load . . . watts 

128. Hysteresis . . . watts 

129. Eddy currents . . . watts 

130. Total . . . watts 

131. Total I"'R and core loss at 

full load . . . watts 

132. Watts per sq. in. radiating sur- 

face, full load 



Estimated rise of temperature 
at full load . . . C. 

Friction of windage and bear- 
ings . . watts 

Field Coils. 

Radiating surface (heads) . . . 
sq. in. 

Radiating surface (periphery) 
. . . sq. in. 

Total radiating surface . . . 
sq. in. 

1*R shunt coils and rheostat 
. . . watts 

7 2 R series . . . watts 

Total I 2 R . . . watts . . , % 

Watts loss per sq, in. . . . radi- 
ating surface 

Estimated rise of temperature 
at full load . . . C. 





Commutator, 




143. 


Brush friction . . . 


watts 


144. 


Brush contact - , 


watts 


145- 


Other commutator 
watts 


losses 



MAGNETIC. 

146. 0« at open circuit 

147. rt at full load 

148. Leakage coefficient 

149. 4>f at open circuit . . 4>f at 

full load 

150. Ampere turns required for 

shunt 

151. Ampere turns required for 

series 



250 RELUCTANCES AND AMPERE TURNS PER POLE. 



2 

K 

H 


a 
u 

K 


Full 
Load. 








.s 


1* 


^0 










j 










en 
i-l 

S3 

CJ 

































i 








T3 O 






% u 












fr 




< 


CO 














h! 














•5 -2 '2 
2 - ^ 




J 














H 


Hi 


«*' 












c3 3 <u 


Z 




CO 












2 *5 3 
* S * S 


Q 
















g <u c "g 


u 

h 

W 
















-2 > 2J rt 

*— O IT* i ~ l 

+- ^ .2 
Is ° ^3 




g 




z 

<! 




CO 












s 


Q 
< 












>, c .a £ 




C 
O 














<u e ^ 

,h <u O ri 
CC O 








£ 


K* 














w 

CO 






It 
O 

H 


u 
z 
w 


W U W J 

ro •**• to vO 

VO ^O VO MD 














M M M M 




■avo'i 




£ 


ON 








u 
fa 




•avo^ 






-rmj 








w 






H 












fl 






< 




O 




Cfl 


CO 








Cfl ^ 


w 






O 




j2 £ c 


5 H 
3 s 






D u 




0) 




| .s ? 

C £« r2 




O 1 Z 

COrS 










c/i 






S 




c5 




"S 5 *p 

« 3 ^ 






z 
a 


w 

1 




3 




•^ j , , . ts 


i-l 




X) 




^ 5 rt 8 




z 




<D 










"3 

0- 
<v 
u 








T3 
QJ 

CD «, CJ "^ 






h £ gj <u 


w 




S 3 * 
^ ^ -C v 
aS aj > 

Z Z £ s 

H H W u 






£ £ 










E. " ^ c 


3 

0) 
0) 










a, 

B 




Os O >-i n 






< 




H, H, 






m fO"t vovo r-~ 


00 










LO i_o 10 i-O >-0 >-0 










M M HI HI HI HI 









TESTS. 



25 



CHAPTER XIV. 



TESTS. 



122. Determination of a (B-OC Curve. — The most exact 
laboratory method of finding this curve is by the ballistic 
galvanometer or ring method. Fig. 1 79 shows the arrange- 
ment of apparatus for this 
method. X is the test piece in 
the form of an annular ring, hav- 
ing a mean circumference of / 
centimeters and a radial cross- 
section of a sq. centimeters. It 
is wound uniformly with n prim- 
ary turns of wire. Over these 
three or four secondary test turns 
of wire lead off to the ballistic 
galvanometer G. A series cir- 
cuit is formed of a storage bat- 
tery or other suitable source of 

E. M. R, B, a variable resistance R, the primary coil of 
the test piece, an ammeter A, and a com mutating switch C. 
The last is used for reversing the direction of the current 
in the primary coil. 

If R be adjusted to give a current I, then the magne- 
tizing force, or 5C, by § 2 1 is represented by the formula 

4.7ml 




5C = 



10/ 



252 DYNAMO ELECTRIC MACHINERY. 

If now the current be suddenly commutated, all the lines of 
force will be withdrawn, and as many more set up in the 
opposite direction. Each of these lines will induce, upon 
commutation, a pressure in each turn of the test coil. 
This induced E.M.F. furnishes a means of measuring the 
flux of lines in the test piece or the flux density (B. By 
application of the formula given in § 16, one may obtain 
the expression for this quantity, 



Where 

J? t = the resistance of the test coil, the galvanometer, and 

the secondary circuit ; 
a = the area of a radial section of the test piece in 

square centimeters ; 
n 2 = the number of turns in the test coil ; 
k = a constant of the galvanometer; and 
= the throw of the galvanometer which accompanies the 
commutation of the primary current. 

Though the most accurate method, the ring method is 
not generally employed in commercial practice because of 

the cost and the time re- 
quired in preparing a test 
piece. 

The divided-bar method 

admits of the use of a bar 

'.mean length of iron of ordinary shape as 

- TEST PIECE J 

ballist,c a test piece. This is cut 

.ALVANOMETER IT 

Fi s- l8 °- into two pieces. A heavy 

wrought-iron yoke, Fig. 180, has a magnetizing coil wound 
inside of it. Through snug-fitting holes in the ends of 
the yoke, the two halves of the test piece are inserted, 




TEST 
COIL 



TESTS. 



253 



one being secured, and the other being fitted with a 
handle. The test coil is so mounted on springs as to 
fly suddenly to one side when the test pieces are slightly 
separated by a pull on the handle. It thus cuts all 
the flux in the piece, and affords a means of measuring 
it. The yoke is so massive, and has such a small reluc- 
tance as compared with that of the test piece, that the 

formula JC = ^-^ — is practically true, where / is the mean 
10 / 

length of the test piece which is traversed by magnetic 

lines. For (B the formula is twice what it was in the 

ring method, since the test coil cuts the flux but once, or 



(B = 



io 8 ^ 



an* 



The method of reversals could be used equally well with 
this apparatus, requiring the formula used in the ring 
method. 

The permeameter is a machine for measuring the flux in 
a test piece by measuring the force necessary to detach it 
from another part of the magnetic cir- 
cuit. Fig. 1 8 1 shows in simple such a 
machine. The magnetizing force is 
supplied by the coil C the same as in 
the divided bar method. The test coil 
and galvanometer are done away with. 
The bottom of the yoke Y is surfaced 
to receive flatly the end of the test rod 
T. When the proper current is flow- 
ing in the coil, the force necessary to 
separate the test piece from the yoke is found by means of 
the spring-balance 5. Since the force required to break 




Fig. 181. 



254 DYNAMO ELECTRIC MACHINERY. 

any number of lines of force varies as the square of that 
number, it is easy to calculate the flux, and since 

<J> = area X 03, 

the magnetic density is readily found. The value of 3C is 
obtained as in the preceding case. 

123. Determination of the Ballistic Constant. — The 

standard condenser affords the most convenient and accu- 
rate means of determining the constant of a ballistic gal- 
vanometer. If the capacity of the condenser C, and the 
voltage at which it is charged E, be known, then the quan- 
tity of electricity that will flow when the circuit is closed 
through the galvanometer is also known. It is equal to 
EC. By observing the galvanometer throw 0, the value of 
the constant k is determined from 

Q = EC= kO. 

The coil of a d'Arsonval ballistic galvanometer moving in 
its field has an E.M.F. induced in it, which tends to send 
a current in a direction opposite to that of the current that 
produces the throw, and which, therefore, shortens the 
throw or damps the galvanometer. The magnitude of this 
damping current depends, of course, on the resistance of 
the galvanometer circuit ; hence the constant k should be 
determined with the same external resistance in the gal- 
vanometer circuit as there will be when the test for the 
value of 03 is being made. 

To accomplish this an arrangement of apparatus such as 
is shown in Fig. 182, may be employed, the particular fea- 
ture of which is the quadruple contact key. This key is 



TESTS. 



255 



normally held up against a contact. In this position the 
galvanometer circuit is open, and the condenser is in series 
with a charging battery. 
As the key is pressed down, 
three things occur. First 
the battery circuit is broken, 
then the condenser is dis- 
charged through the gal- 
vanometer, and lastly the 
galvanometer circuit is 
closed through an appro- 
priate amount of resistance 
in the rheostat. 

The ballistic constant may also be determined by the 
use of a long solenoid, with a few turns about its center 
for a test coil. A series circuit is formed (Fig. 183) of 
a battery, a variable resistance, an ammeter, the solenoid, 



<2> 



RHEOSTAT 



Fig. 182. 




Fig. 183. 



and a key. The ends of the test coil are attached to the 
galvanometer through proper resistance. On closing the 
circuit the current / sets up a field at the center of the 
solenoid, whose intensity, 



3C = 



\TtnI 
10/ ' 



256 DYNAMO ELECTRIC MACHINERY. 

where / is the length of the solenoid in centimeters, and n 
the number of primary turns. 

If A be the area in square centimeters of a cross-section 
of the solenoid perpendicular to the axis, then 

<j> = 3CA. 

If n 2 , be the number of turns in the test coil, and R be 
the resistance of the galvanometer circuit in ohms, then 
upon closing the circuit and upon establishing the flux <£, a 
quantity of electricity will pass around the secondary cir- 
cuit which is equal to 

n 2 <j> 4 irnn 2 IA 



Q = kO = 



io 8 ^ 10 V?/ 



where is the throw of the galvanometer corresponding to 
the current /. Therefore, 



k = 



4 imn 2 IA 



icPRlO 

If the solenoid be less than ten diameters long, this result 
is not accurate, owing to the influence of the ends of the 
solenoid upon the value of 3C. 

There have been described numerous methods for deter- 
mining k which depend upon a constant and definite inten- 
sity of the earth's magnetic field. Nowadays the fact 
that the earth's field is constantly changing, both in direc- 
tion and magnitude, due to the prevalence of iron and steel 
buildings, and the extensive use of electric currents for 
trolley, lightning, and other purposes, makes these methods 
practically worthless. 

124. Determination of the Hysteretic Constant. — The 

hysteresis curve for any sample of iron may be found most 
accurately by the step-by-step method. The arrangement 
of apparatus is in all respects similar to that of Fig. 179, 



TESTS. 257 

save that the rheostat R must be so designed that the cir- 
cuit does not open, even for an instant, 111 passing from 
one resistance point to another. The method of operation 
is as follows : The rheostat is set for a maximum current 
strength which is determined by means of an ammeter. 
The rheostat handle is then quickly moved back one point. 
This reduces the current and the dependent magnetizing 
force proportionately. There is an accompanying decrease 
of flux in the sample. This decrease is determined by the 
galvanometer throw, the formula being as before, 

• ^ io 8 i? , . 
Change in 05 = kd. 

The ammeter current is again read ; and, as soon as the 
galvanometer comes to rest, the resistance is increased by 
another step, and the throw of the galvanometer is observed. 
After the current has been reduced step by step to zero, it 
is then commutated, and increased by steps until the maxi- 
mum magnetization is obtained in a direction opposite to 
that at the beginning. The current is again cut down by 
steps to zero, is afterwards commutated for a second time, 
and is again increased until the magnetic condition of the 
iron which prevailed at the start is again attained. Giving 
to (& a plus or minus sign, according to the direction of the 
galvanometer throw, the algebraic sum of all the changes 
of (ft must equal zero. Therefore the algebraic sum of 
all the galvanometer throws should equal zero. A simple 
addition serves as a check on all observations. In practice, 
the sum of the plus throws may differ from the sum of the 
minus ones by three per cent without seriously affecting the 
final result. Having the maximum value of (ft, the (B's cor- 
responding to each can readily be found by subtracting the 



258 



DYNAMO ELECTRIC MACHINERY. 



changes in (B from the maximum (B. Upon plotting a cyclic 
curve of the various values of & and the corresponding 
values of 5C, one obtains a hysteresis loop, as in Fig. 184. 
The area of this loop in (B3C units, when divided by 477-, gives 
the ergs loss of energy in carrying one cubic centimeter of 
the iron under test through a cycle of magnetization be- 
tween the limits of + (B max and — ®> max . According to the 
Steinmetz formula, 

h = r,<&* M 
where h is the loss by hysteresis in ergs per cycle per cubic 
centimeter. Hence, to find the hysteresis constant y of 
the sample used in the foregoing test, one uses the formula 

where A h = area of hysteresis curve expressed in (B3C units, 
and V=. volume of iron in cubic centimeters. 




Fig. 184. 



TESTS. 



259 



A much less laborious method of measuring 77, and one 
which does not introduce the errors attending the measure- 
ment of the area of a curve, is the wattmeter method. Since 
the iron to be tested is generally for use in alternating cur- 
rent apparatus, this method has the additional advantage 
that the test occurs under the conditions which the iron 
will meet in its working. 

If the ring be made of annular stampings of sheet metal 
well shellacked before assembling, then the loss due to eddy 
currents will be negligible. The arrangement of apparatus, 
shown in Fig. 185, consists of a source of alternating cur- 
rent, a wattmeter, an alternating current ammeter, an alter- 
nating current voltmeter, and the test ring, all connected 
as shown. 





Fig. 185. 

Let R = the resistance of the coil on the test ring ; 
n = number of turns in this coil ; 
W = the watts indicated by the wattmeter ; 
/= the current indicated by the ammeter ; 
E = the pressure indicated by the voltmeter ; 
V— the volume of the iron in cubic centimeters ; 
A = the area of a radial cross-section in square centi- 
meters ; then, assuming the current to be sinus- 
oidal, and of frequency /cycles per second, 



\o\W -PR) 

*>= vj 



(\l2irnfAV- 



26o 



DYNAMO ELECTRIC MACHINERY. 



Ewing's machine for hysteresis tests is shown in Fig. 186. 
Its chief advantage lies in the fact that the test piece needs 
to consist of but half a dozen pieces of sheet iron f /r by 3". 
This test piece is made to rotate be- 
tween the poles of a permanent mag- 
net, which is mounted on knife edges 
on an axis coincident with the axis of 
revolution of the test piece. The re- 
sulting angular displacement of the 
magnet, as marked by a pointer on a 
divided scale, is proportional to the 
hysteresis loss in the specimen. A 
calibration curve is plotted by using 
two different specimens having known 
hysteretic constants. It is found that 
small variations in the thickness of the test piece do not 
affect the results, and that no correction need be made 
for such variations. The machine yields but comparative 
results. 




Fig. 186. 



125. Determination of Leakage Coefficient. — The ratio 
of the total flux generated by a field magnet to the flux 
passing through the armature, that is, the leakage coefficient, 
which is always greater than unity, may be found with an 
arrangement of apparatus as shown in Fig. 187 where the 
machine is a yoke-wound bipolar. A test coil of a few 
turns is passed around the center of the field magnet, and 
through it all the lines generated may reasonably be as- 
sumed to pass. A similar coil is passed around the arma- 
ture, in a plane perpendicular to the direction of the flux, 
through which all the armature flux must pass. In the 
case of a small machine, normal exciting current is passed 



TESTS. 



26l 



through the field magnet, with arrangements for rapid com- 
mutation. In this case, if one test coil have its ends 
attached to a galvanometer or a low-voltage voltmeter, and 
if the current in the field coil be commutated, a deflection, 
which is proportional to the change of flux, will be observed. 
The same will happen if the other coil have its terminals 
connected to the galvan- 
ometer. If f and 6 a be 
the deflections observed 
with the field and the 
armature test coils re- 
spectively, then, as be- 
fore, 

io 8 7? 



TO GALVANOMETER 



* 



kQ f , and 



2 ii 



r 


CD 


"^ 












Willi 








-><~^ — ,+ 



Fig. 187. 



If the two test coils be constructed alike as regards number 
of turns and resistance, then the values of R, n 2 , and k are 
the same in both equations, and we have the leakage coef- 
ficient 

fa _ fy 

Hence the ratio of the galvanometer throws gives the co- 
efficient without further calculation. 

This method may be employed to obtain the flux in any 
part of the magnetic circuit, and it serves to locate the 
points of greatest leakage. It may also be modified to apply 
to any type of machine. In the case of large machines, 
whose field currents cannot be commutated, a cyclic in- 
crease and decrease of exciting current can be produced 
by means of cutting out and in of resistance in the field 



262 



DYNAMO ELECTRIC MACHINERY. 



circuit. Even then the time constants of large field coils 
are so great as compared with the period of swing of ballis- 
tic galvanometers, that the method is impracticable. 

126. Magnetic Distribution in the Air Gap. — Since ar- 
mature reactions distort the magnetic field, it is desirable 
to know the actual distribution of the flux. This may be 





determined by the use of a pilot brush, as shown in Fig. 
188. A voltmeter is connected between one of the main 
brushes and the pilot brush, and the latter is moved 
through equal angular intervals until the opposite brush 
is reached. The difference in the voltage of any two 
consecutive readings is proportional to the magnetic flux 
within the angular distance moved over between those 
two readings. 

Two pilot brushes may be used as in Fig. 189. In this 
case the voltage is proportional to the flux corresponding 
to the angular distance between the two brushes. By 



TESTS. 



263 



applying these brushes at successive intervals through 
180 the flux distribution can be determined. 

127. Measurement of Resistance a. By voltmeter alone. 

For insulation resistances, or any resistances lying between 
about 5000 and 100,000 ohms, a fairly accurate result may 
be obtained by arranging the unknown resistance x and a 
0-150 voltmeter in series with a source of constant potential 
of about 1 1 5 volts. The reading is noted. The resist- 
ance is then short-circuited and the deflection & noted. If 
R be the resistance of the voltmeter, then 



R. 







Maximum accuracy is obtained when x = R. 



MW1AWM 





71 n 

Fig. 190. 

b. By the Method of Wheatstone' s Bridge. If an un- 
known resistance x, two known resistances a and b, and 
a known adjustable resistance R be connected as shown 
in Fig. 191, with a galvanometer G and a battery cell B, 
a Wheatstone's bridge is formed ; and, if the resistance R 
be so manipulated as to prevent a flow of current through 
the galvanometer, then the following relation is true : 

a : b : : R : x. 



264 



DYNAMO ELECTRIC MACHINERY. 



It is usual to make the ratio a : b equal to some multiple 
or submultiple of 10. In this case the value of X is read 
directly from R with the decimal point suitably placed. 

This method permits of great accuracy. 

C. By ammeter and voltmeter. Resistances of ordinary 
magnitudes are most conveniently measured by measuring 
the pressure impressed on the resistance and the current 
caused to flow thereby. This is the most practical method 
for finding the resistances of armature and field-windings 
of dynamos. 

It is a method so rapid that the value of hot re- 
sistances may be found, and fields can be measured even 







Fig. 192. 

while the machine is in operation. Fig. 192 shows an 
arrangement of apparatus for measuring the resistance of 
an armature, including the brush and contact resistances. 
If I be the ammeter reading, and E be the voltmeter read- 
ing, then by Ohm's law 

R 



E 
7 



128. Test of Dielectric Strength In order to test the 

voltage necessary to break down a sample sheet of insulat- 
ing material, the sample is placed between two flat metal- 
lic surfaces, which are connected respectively with the two 
terminals of a high-voltage transformer, whose voltage can 
be varied at will. An air gap between needle-point ter- 



TESTS. 



265 



minals which can be adjusted in length is connected in 
parallel between the two terminals. The distance between 
these points serves to limit the voltage which can be im- 
pressed upon the conductors on each side of the insulating 
material. For small variations of gap length the voltage 
necessary to produce an arc between the needle-points is 













































56 
54 


























































































































52 










































50 
48 
46 




























^ 






































1 






































1 












42 


















































y 
































2 


2. 


A 


i 


■?. 


(3 


2 


S 






3 


2 


3.4 


3 


G 


3 


S 


i 






y 








































/ 








































<2 32 
-30 
§ 28 
2 2C 






































A 




































































































































































22 
20 

18 
16 
14 

12 
10 

8 
6 

4. 


















































































































































































































































































































































































































/ 


/ 














































































< 


) 




1 




1 


. 


5 




8 
I 


ICY 


1. 


1 


.2 


1 


4 


1 


6 


1 


8 


2 



Fig. 193. 

nearly proportional to the length. The following table, 
taken from the Standardization Report of the American In- 
stitute of Electrical Engineers, shows the relation which 
exists between air-gap length and the voltage necessary 
to produce a disruptive discharge. The relations are also 
exhibited in the curve of Fig. 193. 



266 



DYNAMO ELECTRIC MACHINERY. 



TABLE OF SPARKING DISTANCES IN AIR BETWEEN 
OPPOSED SHARP NEEDLE-POINTS, FOR VARI- 
OUS EFFECTIVE SINUSOIDAL VOLTAGES, 
IN INCHES AND IN CENTIMETERS. 



KlLOVOLTS. 


Distance. 


KlLOVOLTS. 


Distance. 


Sq. Root of 
Mean Square. 


Inches. 


Cms. 


Sq. Root of 
Mean Square. 


Inches. 


Cms. 


5 

IO 

J 5 
20 

25 
3° 
35 
40 

45 
50 


O.225 
O.47 
O.725 
1.0 

i'-3 

1.625 

2.0 
2.45 
2-95 
3-55 


0.57 
1. 19 
I.84 
2.54 

3-3 
4.1 

5- 1 
6.2 

7-5 
9.0 


60 
70 
80 
90 
100 

no 
120 

130 

140 

150 


4.65 

5-85 

7-i 

8-35 
9 .6 

10.75 
11.85 

12.95 
13-95 
15.0 


1 1.8 
14.9 
18.0 
21.2 
24.4 

27-3 
30.1 

3 2 -9 
35-4 
38.1 






In carrying out the test, the needle-points are adjusted 
at a certain minimum distance apart. The voltage im- 
pressed upon the terminals is raised until a spark passes 
between the points. The air gap is then increased in 
length, and the operation repeated until the sample breaks 
down, and the spark passes through it instead of across 
the air gap. The break-down voltage is then taken from 
the table or curve corresponding to the last position of the 
needle-points. 

The sample should project considerably beyond the 
edges of the compressing surfaces. Owing to surface 
leakage a spark will pass over a very much greater distance 
of the surface of an insulator than it will in free air. 

For the purpose of obtaining a voltage any form of high- 
potential transformer may be used, the primary being sup- 



TESTS. 267 

plied by an alternating current. Fig. 194 illustrates a 
10,000 volt transformer manufactured for this purpose by 
the General Electric Co. Its core is of the H type, and 




Fig. 194. 



upon one branch of it is wound the low-tension circuit, 
while upon the other is wound the secondary, consisting of 
four coils, each wound and insulated independently. The 
four coils are assembled upon a sleeve of heavy insulating 



268 



DYNAMO ELECTRIC MACHINERY. 



material. The transformer is immersed in oil, and its 
primary is wound in two parts so that it may be used upon 
a 52 or a 104 volt circuit. The adaptation to either of 
these circuits is rendered possible by means of a porcelain 
series multiple connection board which is placed inside the 
inclosing case. On the top of the apparatus is a box with 
a glass window, which incloses a micrometer spark gap, 
which is connected in shunt across the high-potential 



TO 
TEST 



SECONDARE 




1 



5" 



i 



Fig. i95. 

terminals. This box or cover carries 
four long contact studs which fit into 
sockets. In the transformer box the 
apparatus is so arranged that the lifting 
up of this cover for the purpose of ad- 1 u ~"' ' 
justing the spark gap entirely discon- 
nects the spark gap from the high-potential circuit. The 
connections of this apparatus to a sample under test are 
shown in Fig. 195. 

This apparatus may also be employed in determining 
whether a given sample of insulation will withstand an im- 
pressed electromotive force without breaking down. The 



TESTS. 



269 



length of the gap is set so as to represent the value of the 
prescribed electromotive force, and the sample is subjected 
to the pressure which maintains a spark across the gap. 
In case of break-down the spark at the gap will cease. 

In case it is desired to test the dielectric strength of the 
sample at some other than normal temperature, the sample 
may be pressed between two surfaces of the apparatus 
shown in Fig. 196, which was described by Mr. Charles F. 




Fig. 196. 

Scott. Two carefully faced blocks of cast iron are re- 
cessed so as to receive coils D and D r of asbestos- wound 
wire. These coils are supplied with alternating current 
which raises the temperature of the disks by means of 
eddy currents and hysteresis losses. Upon shutting off 
the current, the disks and insulating material soon assume 
a uniform temperature, which can be measured by means 
of a thermometer whose bulb is inserted in a hole in the 
upper disk. The two disks are made the terminals of the 
high-tension circuit. Connections with the circuit which 
is used for heating purposes must of course be removed 
during the test. 



2JO DYNAMO ELECTRIC MACHINERY. 

The report of the committee on standardization of the 
American Institute of Electrical Engineers gives the fol- 
lowing : " The dielectric strength or resistance to rup- 
ture should be determined by a continued application of an 
alternating E.M.F. for five minutes/* 

" The test for dielectric strength should be made with 
the completely assembled apparatus and not with its indi- 
vidual parts, and the voltage should be applied as fol- 
lows : ist, Between electric circuits and surrounding 
conducting material, and 2d, between adjacent electric cir- 
cuits where such exist." 

The report further recommends for apparatus, not in- 
cluding switchboards and transmission lines, the following 
testing voltages : — 

RATED TERMINAL VOLTAGE. CAPACITY. TESTING VOLTAGE. 

Not exceeding 400 volts Under 10 K.w. . . 1000 volts. 

Not exceeding 400 volts 10 K.w. and over . 1500 volts. 

400 and over, but less than 200 volts Under 10 k.w. . . 1500 volts. 

400 and over, but less than 800 volts 10 k.w. and over . 2000 volts. 

800 and over, but less than 1200 volts Any 35 00 volts. 

1200 and over, but less than 2500 volts Any 5000 volts. 

( Double the normal 

2 soo and over Any . . < 

J J ( rated voltages. 

Synchronous motor fields and fields of converters started 

from the alternating current side 5000 volts. 

The values in the table above are effective values, or square roots of 
mean square reduced to a sine wave of E.M.F. 

When machines or apparatus are to be operated in series, so as to em- 
ploy the sum of their separate E.M.F.% the voltage should be referred to 
this sum, except where the frames of the machines are separately insulated 
both from ground and from each other. 

129. Determination of the Magnetization Curve of a 
Shunt-Dynamo. — To find the relation between the exciting 
current and the no-load terminal volts of a shunt machine, 



TESTS. 



271 



excite tne shunt fields, Fig. 197, from an external source, 
first passing the current through a variable resistance and 




Fig. 197. 

an ammeter. Run the machine at a constant speed through- 
out the test. If a voltmeter be placed across the armature 
terminals a pressure can be read corresponding to each 
exciting current, and a curve can be plotted using volts as 
ordinates and amperes as abscissae. Because of residual 




Fig. 198 

magnetism there are some volts with no exciting current, 
and hence the curve, Fig. 198, does not pass through the 
origin. 

If the voltmeter be read while the current is increasing 
by steps to the maximum, and again while the current is, 



272 



DYNAMO ELECTRIC MACHINERY. 



decreasing, step by step, the two curves will not coincide ; 
the descending curve will lie above the other as in Fig. 199. 
This is because of the hysteresis or magnetic retentivity of 
the iron of the magnetic circuit. 




Fig. 199. 

130. Efficiency of Dynamos and Motors The efficiency 

of these machines can be determined by any one of the 
following methods : — 

a. Run the machine at its proper speed as a separately 
excited motor. Let the excitation be normal. By means 
of ammeter and voltmeter readings determine the electrical 
input, the motor having no load upon it. The arrange- 
ment of apparatus is shown in Fig. 200. The power put 
in represents the PR losses in the armature and the field 
plus the losses which are generally considered as constant 
at al 1 loads. These constant losses are those due to fric- 
tion, hysteresis, Foucault currents, and windage. They 
are equal to the no-load input minus the no-load PR arma- 
ture and field losses. The PR losses can be calculated at 



TESTS. 



273 



any useful load. The efficiency at that load is equal to 
the load divided by the load plus the sum of the constant 



+ SERVICE MAIN 




ADJUSTABLE' 
RESISTANCE? 



— SERVICE MAIN 

Fig. 200. 

losses and the load PR losses. The machine at the time 
of no-load test should have the same temperature as it 
would have under the load for which the efficiency is being 
calculated. 

b. Run the machine as a motor at its 
rated speed and temperature. Measure 
the electrical input by a voltmeter and an 
ammeter. Measure the mechanical output 
by a Prony brake. Then the efficiency, 

watts at brake 

rj = ; 

watts input 

There are many kinds of brake or ab- 
sorption dynamometers that may be used 
for this test. The most satisfactory one 
for motors of small size is the strap-brake shown in 
Fig. 201. A piece of leather belting and two spring 
balances are all that is necessary. The formula for the 
absorbed power is, 




Fig. 201. 



Watts = 



2irrV{P -P') 



X 746, 



where r 



33000 
the radius of the pulley in feet ; 



274 



DYNAMO ELECTRIC MACHINERY. 



V 

(P - p>) . 

pounds. 



number of revolutions per minute ; 

the difference of the two-scale readings in 



Fig. 202 shows a form 
of brake applicable to lar- 
ger machines. The for- 
mula for the power ab- 
sorbed is, 




Fig. 202. 



Watts = 



lirrVP 

33000 



x 746, 



where r is the perpendicular distance from the center of 
the pulley to the line of action of the scale in feet, P the 
scale reading in pounds, and V the number of revolutions 
per minute. The brake should be so poised as to give no 
reading on the spring at no load. 

The brake may be made with a metal strap having 
spaced blocks on its under surface that screw down against 
the wheel, and for the spring balance one may use a plat- 
form scale having a prop extending to the lever arm of the 
brake. For large machines the heat generated by the 
absorption of considerable power at the face of the pulley 
causes an excessive rise of temperature. It is necessary 
to find some means of carrying the heat away. This is 
generally accomplished by flanging the inside of the brake- 
wheel, forming a trough in which water is kept running. 
Centrifugal force throws the water against the internal 
circumference of the wheel and prevents spilling. The 
water is removed either by a properly placed scoop, or it 
may be allowed to boil out. 

c. A convenient method of finding and separating the 
losses of a machine is one which makes use of a rated 



TESTS. 275 

motor, i.e., a motor whose mechanical output is known for 
any given electrical input. By reading the volt-ampere 
input of the motor the power expended on the machine to 
be tested can be found. Run the machine by the rated 
motor at the proper speed. If the brushes be removed 
from the machine, and no current be flowing in the field 
coils, then the power expended on it is the loss due to 
friction at the bearings and to windage. Now let the 
brushes be set, then the power expended is the loss due to 
windage, bearing friction, and brush friction. By sub- 
traction the brush-friction loss is found. This is greater, 
particularly in small machines, than is generally supposed. 
Now let the fields be separately excited by the normal 
current, and the losses due to hysteresis and eddy currents 
are included in the power expended on the machine. 
From a knowledge of the hot resistances of the machine, 
one can calculate the I 2 R loss for any useful load in both 
armature and field windings. This useful load divided by 
the sum of the useful load and all the losses, gives the 
efficiency of the machine at that load. 

d. The methods a, b, and c all require some outside 
electric power. This requirement can be avoided by the 
use of a transmission dynamometer to measure the power 
input of any machine, and the power output can be read by 
a voltmeter and an ammeter. This method is seldom re- 
sorted to, since transmission dynamometers are often unre- 
liable, they are expensive to set up, and some forms have 
but a limited power range. 

Professor Goldsborough has recently devised a very 
ingenious dynamometer which consists simply of a coiled 
or helical spring with the center line of the helix corre- 
sponding with the center of the shaft. This spring con- 



276 DYNAMO ELECTRIC MACHINERY. 

nects the driving and driven members. Readings are made 
by means of two instantaneous contact points mounted, 
one on the driven and one on the driving-shaft, which are 
connected in series with each other and with a battery 
and telephone receiver. As the spring becomes deflected 
by a load, the contact on the driven shaft falls back, 
and the corresponding brush must be set back by the 
same angle in order to obtain a click in the telephone. 
This angle is a direct measurement of the torque, and can 
be calibrated at standstill. 

e. The efficiency of direct connected units can be found 
by using the indicator card from the steam-engine to deter- 
mine the power input, and by using a voltmeter and an 
ammeter for determining the electrical output. Even if 
the engine losses were exactly known, the measurements 
yielded by an indicator card are hardly exact enough to 
afford a fair basis for testing the efficiency of a generator. 
In other than direct-connected units it is not frequent that 
one finds a generator driven by an engine that does no 
other work. 



INDEX. 



Air-gap, 69, 81, 85. 

distribution in, 262, 87. 
Ampere-turns, 21, 245. 
Angle of lag or lead, 82. 
Armatures, 34, 47, 51. 

Back-turns, 82. 

Bearings, 64. 

Boosters, 216. 

Box, starting for motors, 169. 

Brake on motors, 179. 

prony, 273. 

solenoid, 182. 

strap, 180. 
Brushes, 60, 61, 86, 90. 

Candle-power of arc lamps, 130. 
of incandescent lamps, 103. 
Capacity of a dynamo, 36. 
Cauteries for surgeons, 215. 
Chord winding, 52. 
Circuits, divided, 7, 8. 
Coefficient : 

economic, 93. 

of series dynamo, 97. 
of compound dynamo, in. 
of shunt dynamo, 100. 
of conversion, 93. 
of magnetic leakage, 73, 260. 
of self and mutual induction, 

17, 18. 
of temperature, 5. 



Coercivity, 28. 
Coil: 

armature, 46, 49. 

compound, 71, 246. 

field, 71. 

formed, 55. 
Collection of currents, 61. 
Commutation, 84. 
Commutator : 

construction of, 59. 

losses at, 59. 

principle of, 32, 77. 

segments of, ^^. 
Compounding : 

in motor, 174, 224. 

windings for, 71, 11 1. 
Conductivity, 5. 

of copper, 6. 
Connections : 

for combined output, 218. 

of motors, 223. 

series dynamos in series, 221. 

shunt dynamos in parallel, 220. 

shunt dynamos in series, 221. 

cross in armature, 48. 
Constant : 

determination of ballistic by 
condenser, 254. 

determination of ballistic by 
long solenoid, 255. 

determination of ballistic earth's 
field, 256. 



277 



278 



INDEX. 



Constant : 

determination of hysteretic, 256, 
259, 260. 

hysteretic, 29. 
Contact of brushes, 60. 
Controller : 

for mill motors, 206. 

for street-railway motors 197. 

fingers for, 198. 

reversing lever, 199. 

wipes, 198. 
Core, armature, 37, 57. 

field, 67. 
Correction for tooth densities, 243, 
Cross-magnetization. 80. 
Current density, 72. 

in armature, 234. 
Currents, foucault or eddy, 38, 90. 
Curve, B-H., 24, 25, 244. 

characteristic, 96, 98, 101, 174. 

determination of B-H, 251. 

determinization of magnetiza- 
tion, 270. 

Demagnetization, 82, 246. 
Density of flux, 22. 

in air-gap, 233. 

corrections for, in teeth, 243. 

determination of distribution 
of, 262. 

in field, 131, 241. 
Design, data sheet for, 247. 

different methods of, 232. 

of armature, 235. 

of field, 241. 

preliminary assumption for, 233 

specifications for, 233. 
Diameter of armature, 237. 

of shaft, 64, 119. 
Dielectric test of insulation of, 268. 

test of strength of, 264, 270. 



Direction of induced E. M. F., 16. 

of rotation of a motor, 161. 
Drop, 4. 
Drums, 34, 51. 
Dynamo : 

arc lighting, 129. 
Brush, 134. 
Westinghouse, 141. 
Wood, 143. 
Excelsior, 147. 
Ball, 149. 

Thomson-Houston, 151. 
Western Electric Co., 156. 
definition of, 31. 
direct-driven, 118. 

Bullock Electric Co., 127. 
Crocker-Wheeler, 123. 
General Electric Co., 121. 
Lundell, 88, 124. 
Sprague Electric Co., 88, 124. 
Westinghouse Co., 119. 
Dynamotors, 208. 

armature, reaction in, 208. 
for Bullock teazer system, 209. 
for electrometallurgy, 211. 
as rotary equalizer, 212. 
for telegraphic work, 213. 
Dyne, definition of, 1. 

Efficiency, 92. 

of compound dynamo, 112. 

of compound motor, 165. 

determination of, 272. 

of direct connected units, 276. 

of motors for automobiles, 202. 

of shunt motors, 166. 

of series motors, 166. 
E. M. F. : constant supply of, 
103. 

counter in motors, 163. 

direction of induced, 16. 






INDEX. 



279 



E. M. F. : 

in shunt dynamos, 100. 

in separately excited dynamos, 

95- 

in series dynamos, 97. 

of induction, 14. 

of self and mutual induction, 
17, 79, 84. 

of eddy currents, 38. 

principle of production of, in 
armature, 32. 

unit of, 3. 
Energy, 1, 89. 
Equalizer, bus, 222. 

rotary, 212. 
Erg, 1. 
Excitation, mutual, 222. 

separate, 94. 

of fields, 69. 

Fall, of potential, 4. 
Feeding-points, 104. 
Field, magnetic, 12, 21, 67. 
Fleming's rule, 16. 
Fluctuation of E. M. F., 34. 
Force, magnetizing, 22. 

magnetomotive, 20, 26, 245. 

units of, 1. 
Foot-pound, 1. 

Frequency of commutation, 84. 
Friction of bearings, 89. 

of brushes, 60. 
Fuses, 10. 

Gap, air, 69, 81, 85, 233. 
Generators [see dynamo], 31. 

Heat of current, 9. 
Heating of armatures, 39. 
Holders for brushes, 61. 
Hysteresis, 27. 



Hysteresis : 

losses by, 89. 

Inductance, 17, 18, 84. 
Induction : 

electro-magnetic, 13. 

mutual, 18. 

self, 17, 79. 
Inductors, 34, 45. 
Input, 92. 
Intensity of magnetic field, 12. 

Jig, for filing brushes, 62. 
Joints in magnetic circuit, 76. 
Joule, definition of, 1. 

Lag, 82. 

of brushes in a motor, 165. 
Lamination, 38. 
Law of Steinmetz, 29. 
Lead, 82. 
Leakage, magnetic, 73. 

determination of, 260. 

in compound motor, 175. 
Length of armature, 237. 

of active conductor, 235. 
Lines of force, 12. 
Link, fuse, 10. 
Losses, 92, 93. 

armature, 238. 

commutator, 59. 

fixed, 167. 

I 2 R, 9. 

in operation, 89. 

variable, 167. 
Lubrication, 65. 

Magnet, field, 35, 69. 
Magnets, 70, 96. 
Materials, insulating, 6. 
magnetic, 24, 68. 



280 



INDEX. 



Matthiessen, standard of, 6. 
Measurement of temperature rise, 4: 
Melting of commutator bars, 83. 
Meters, recording watt-hour, 182. 
Mica, 7, 59. 
Mil, circular, 6. 

•foot, 6. 
Motor, brake, 179. 

compound wound, 174. 

counter E. M. F. of, 163. 

direction of rotation of, 161. 

for railways, 187. 

for automobiles, 200. 

for mills, 203. 

principle of, 161. 

rated, 274. 

series, 185. 
Motor-generators, 215. 

Oilers, 65. 

Operation, care in, 225. 
Output, 92. 

Over-compounding, in. 
coils for, 246. 

Permeability, 22. 
Permeameter, 253. 
Permeance, 25. 
Plane : 

commutating, 78. 

neutral, 78. 
Point, running on controller, 197 
Pole, magnetic, 12. 
Pole pieces, 67, 75. 
Potential, magnetic, 19. 
Power, lines of, 99. 

of electric current, 8. 

unit of, 2. 
Pressure, 4. 

Prevention of sparking, 85. 
Process of commutation. 77. 



I Rating of machines, 39. 
Reaction of dynamo armature, 80. 
of dynamotor armature, 208. 
of motor armature, 165. 

Rectifier, compounding, 112. 
Re-entrancy, 46. 
Regulation, arc dynamo, 132. 

hand, 104. 

self, 110-113. 

(see speed). 
Reluctance, 25. 

of dynamo-magnetic circuit, 242. 
Reluctivity, 26. 

Report of Standardization Commit- 
tee of the American Insti- 
tute of Electrical Engineers : 

on efficiency, 92. 

on regulation, 116. 

on spark-gap voltages, 266, 

on temperature elevation. 40. 

on testing dielectric strength, 
270. 
Resistance, armature, 240. 

brush contact, 60, 90. 

measurement of, 263. 
Resistivity, 5. 
Retentivity, 28. 
Rheostats, Carpenter enamel, 108. 

overload, 172. 

packed card, 105. 

starting, 169. 

Ward Leonard Electric Co., 
108. 

Wirt, no. 
Rise of temperature, 41, 91, 60. 
Rockers, 61, 64. 
Rule, Fleming's, 16. 

Saturation of teeth, 85. 
Shafts, 64. 



INDEX. 



28l 



Shape of pole pieces, 86, 

Sheet, data for design, 247. 

Shell, magnetic, 20, 21. 

Shifting of brushes, 85. 

Shoe, pole, 75. 

Short-circuit of armature coil, 226. 

Skewing of field, 81. 

Slip-rings, 31. 

Slotting of poles, 86. 

Solenoid, 21. 

brake, 180. 
Span, polar, 69. 
Spark, voltage of, 266. 
Sparking, 61, 83, 85, 225, 
Spectrum, magnetic, 13. 
Speed, armature, 239. 

motor, 163. 

slow, 178. 

hand regulation of, 175. 

Leonard system of regulation, 
176. 

series resistance regulation, 176. 
Steinmetz, law of, 29. 
Strength, test of dielectric, 264. 
Surface, for radiation in armature, 
240. 

Teazer for Bullock system of speed 

control, 209. 
Teeth, 85. 
Temperature, critical, 25. 

measurement of, 42. 

rise of, 41, 43, 60, 91. 



Tension of brash springs, 62. 
Theory of self-regulation of dyna- 
mos, 113. 
Torque, 2. 

Transformer for high potentials, 266. 
Turns, series, no. 

Unit, absolute and practical, 2. 
mechanical, 1. 
of current, 3. 
of potential difference, 3. 
of resistance, 3. 
pole, 12. 

Velocity, peripheral of armature, 234 
Ventilation, ducts for, 39. 
Vulcabeston, 7. 

Watt, definition of, 2. 
Wheatstone's bridge, 263. 
Windage, 89. 
Winding chord, 52. 

closed-coil, 45. 

cross-connected, 48. 

drum, 51. 

open-coil, 44. 

ring, 47. 

series, 70, 97. 

short-connection, 49. 

shunt, 70, 97. 
Wires, binding, 55. 
Work, 1, 9, 19. 

Yoke, ^6, 67. 



LIST OF WORKS 

ON 

Electrical Science 

PUBLISHED AND FOR SALE BY 

D. VAN NOSTRAND COMPANY, 

23 Murray & 27 Warren Street, 

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ATKINSON, PHILIP. Elements of Static Electricity, with full de- 
scription of the Holtz and Topler Machines, and their mode of operating. 

Illustrated. 12mo, cloth $1.50 

— — The Elements of Dynamic Electricity and Magnetism. l-2mo, cloth. $2.00 
Elements of Electric Lighting, including Electric Generation, Measure- 
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Power Transmitted by Electricity and applied by the Electric Motor, including 

Electric Railway Construction. Third edition, fully revised and new matter 
added. Illustrated. 12mo, cloth $2.00 

BADT, F. B. Dynamo Tender's Hand-book. 70 Illustrations. 16mo, 
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Electric Transmission Hand - book. Illustrations and Tables. 16mo, 

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Incandescent Wiring Hand-book. Illustrations and Tables. 12mo, cloth. 

$1.00 
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BIGGS, C. H. W. First Principles of Electricity and Magnetism. A book 
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BLAKESLEY, T. H. Papers on Alternating Currents of Electricity, for 
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Electric Bells, and all about them. A Practical Book for Practical Men. 

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The Dynamo : How Made and How Used. A Book for Amateurs. Sixth 

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Electro-motors : How Made and How Used. A Hand-book for Amateurs 

and Practical Mjn. Illustrated. 12mo, cloth. $0.50 

BUBIER, E. T. Questions and Answers about Electricity. A First Book for 
Beginners. 12mo, cloth $0.50 

CARTER, E. T. Motive Power and Gearing for Electrical Machinery ; a 
treatise on the theory and practice of the mechanical equipment of power 
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cloth $5.00 

CROCKER, E. B., and S. S. WHEELER, The Practical Management of 
Dynamos and Motors. Illustrated. 12mo. cloth $1.0C 

(ROCKER. F. B. Electric Lighting. Volume I., The Generating Plant. 
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DESISOND, CHAS. Electricity for Engineers. Part I. : Constant Cur- 
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DV ittONCEIj, Count TH. Electro-magnets : The Determination of the 
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DYNAMIC ELECTRICITY. Its Modern Use and Measurement, chiefly 
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Light Arithmetic, by R. E. Day, M. E. 18mo, boards. (No. 71 Van Nos- 
trand's Science Series.) .......... $0.50 

EWING, J. A. Magnetic Induction in Iron and other Metals. Second 
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GORDON, J. E. H. School Electricity. 12mo, cloth $2.00 

GORE Dr. GEORGE. The Art of Electrolytic Separation of Metals 
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HASR.INS, C. H. The Galvanometer and its Uses. A Manual for Elec 
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— — Transformers : Their Theory, Construction, and Application Simplified. 
Illustrated. 12mo, cloth $J.25 

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HOBBS, W. R. P. The Arithmetic of Electrical Measurements. With 
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HOSPITAL IE R, E e Polyphased Alternating Currents. Illustrated. 8vo, 
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INDUCTION COILS : How Made and How Used. Fifth edition. J6mo. 
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INCANDESCENT ELECTRIC LIGHTING : A Practical Descrip- 
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KAPP, GISBERT, C.E. Electric Transmission of Energy and its 
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Dynamos, Alternators, and Transformers. Illustrated. 8vo, cloth . . $4.00 

J5.EMPE, H. R. The Electrical Engineer's Pocket-book ; Modern Rules, 
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KENNEL LIT, A. E. Theoretical Elements of Electro-dynamic Machinery. 
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KILGOUR, M. H., and SWAN, H., and BIGGS, C. H. W. Electri- 
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LOCKWOOD, T. D. Electricity, Magnetism, and Electro-telegraphy. 
A Practical Guide and Hand-book of General Information for Electrical 
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LODGE, PROF. OLIVER J. Signalling Across Space Without Wires : 
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LOEING, A. E. A hand-book of the Electro-magnetic Telegraph. 16mo, 
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HORROW, J. T., and REID, T. Arithmetic of Magnetism and Electricity. 
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Pocket-book of Electrical Rules and Tables, for the use of Electricians and 
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NIPHEE, FRANCIS E., A. M. Theory of Magnetic Measurements, with an 
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OHM, Dr. G. S. The Galvanic Circuit Investigated Mathematically. Berlin 
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PIjANTE, GASTON. The Storage of Electrical Energy, and Researches 
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POPE, F. Ij. Modern Practice of the Electric Telegraph. A Hand-book for 
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POOLE, J. The Practical Telephone Hand-book. Illustrated. 8vo, cloth. 

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RECKENZAUN, A. Electric Traction. Illustrated. 8vo, cloth $4.00 

RUSSELL, STUART A. Electric-light Cables and the Distribution of 
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SALOMONS, Sir DAVID, OT.A. Electric-light Installations. Vol. I. 
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SLOANE, Prof. T. O'CONOR. Standard Electrical Dictionary. 300 
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THOU , C, and JONES, W. H. Telegraphic Connections, embracing 
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TUMLIRZ, I>r. Potential, and its application to the explanation of Elec- 
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TUNXEIiMANN, G. W. de. Electricity in Modern Life. Illustrated. 
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*** A General Catalogue -SO pages— of Works in all branches 
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„*<*, 



FEB 20 1902 



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